A REVIEW ON RECENT PROGRESS OF R&D FOR HIGH … · elevated temperatures, good dielectric properties over wide temperature and frequency ranges, good mechanical properties, and high

22 X.-M. Zhang, J.-G. Liu and S.-Y. Yang

© 2016 Advanced Study Center Co. Ltd.

Rev. Adv. Mater. Sci. 46 (2016) 22-38

Corresponding author: X.-M. Zhang, e-mail: [email protected]

A REVIEW ON RECENT PROGRESS OF R&D FORHIGH-TEMPERATURE RESISTANT POLYMER DIELECTRICS

AND THEIR APPLICATIONS IN ELECTRICAL ANDELECTRONIC INSULATION

Xiu-Min Zhang1, Jin-Gang Liu2 and Shi-Yong Yang3

1School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China3School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China

2Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences,Beijing 100190, China

Received: March 02, 2016

Abstract. High-temperature resistant polymer dielectrics (HTPDs) have been finding great ap-plications in electrical and electronic insulating fields due to their excellent thermal stability atelevated temperatures, good dielectric properties over wide temperature and frequency ranges,good mechanical properties, and high environmental stability, etc. The latest progress of re-search and development on HTPDs has been reviewed in the present paper. The developinghistory, structural features, molecular design, and functionalization for typical aromatic orheteroaromatic HTPDs, including imide polymers [polyimide (PI), polyamideimide (PAI), andpolyetherimide (PEI), polybenzimidazole (PBI), polybenzoxazole (PBO), polyetheretherketone(PEEK), and polyphenylquinoxaline (PPQ) have been summarized. Then, the applications ofHTPDs in electrical and electronic insulating areas were introduced. The future developing trendfor HTPDs was prospected at last.

1. INTRODUCTION

Dielectrics have been becoming one of the mostimportant components for both of electrical and elec-tronic insulating applications because they can pro-tect the equipments or devices from a variety ofdestroying such as dielectric loss, shorting out bysparking, dielectric breakdown, and so on [1-4].According to the chemical nature of dielectrics, theycan be roughly divided into three categories: inor-ganic dielectrics (ceramic, mica, glass, etc.), or-ganic dielectrics (polymer, mineral oil, paper, etc.),and inorganic-organic hybrid dielectrics (polymernanodielectrics, etc.). According to the state ofmolecular aggregation, dielectrics can also be di-vided as gas dielectrics (sulfur hexafluoride, etc.),liquid dielectrics (capacitor oil, etc.), and solid di-

electrics (polymer, ceramic, etc.). Among variousdielectrics, solid polymer dielectrics have been paidincreasing attentions in recent years due to theirexcellent combined properties desired by engineer-ing applications, including good dielectric features,good processing characteristics, and flexible struc-ture designability [5]. Polymer dielectrics can befurther divided into several classes according to theirthermal endurance indices. For example, clear rela-tive thermal endurance index (RTE or RTI) for elec-trical insulating materials (EIM) and electrical insu-lating systems (EIS) have been rated by standardNEMA (National Electrical Manufacturers Associa-tion) classif ications, IEC (InternationalElectrotechnical Commission) and Chinese nationalstandard (GB/T) according to their maximum allow-able operating temperature, as shown in Table 1.

23A review on recent progress of R&D for high-temperature resistant polymer dielectrics and their...

Typical polymer dielectrics corresponding to everythermal class are illustrated in Fig. 1.

Conventional polymer dielectrics, such as poly-ethylene (PE) or polypropylene (PP) intrinsicallypossess excellent insulating properties. However,they are facing great challenges in advanced elec-trical and electronic insulation engineering due totheir limited service temperatures ( 105 °C). In re-cent years, with the development of electrical andelectronic insulating applications to the trends ofhigh power and high current, small and thin profile,high density, and high integrity, the demands to thehigh-thermal-class polymer dielectrics has beenincreasingly emphasized so as to maintain highreliability of the devices operating at high tempera-tures [6,7]. High temperature is usually defined asan operating temperature higher than 175 °C [8].For instance, for the recent aviation and aerospace

(International Electrotechnical Commission, IEC NEMA standard60216) Chinese national standard (GB/T 11021-2014) (for magnet wire)

RTE thermal alphabet thermal alphabetclass class or number

90 T < 105 90 Y 90 O105 T < 120 105 A 105 A120 T < 130 120 E 120 E130 T < 155 130 B 130 B155 T < 180 155 F 155 F180 T < 200 180 H 180 H200 T < 220 200 N 200 200 (K)220 T < 250 220 R 220 220 (M)T 250 250 - 240+ C

250 250

Table 1. Thermal class rating for EIM and EIS.

Fig. 1. Typical polymer dielectrics and their thermal endurance index values.

electrical machine winding applications, the electri-cal insulating system must withstand the high tem-perature environment in excess of 200 °C with life-times of several thousands of hours for the engines[9]. Elevated temperature environment usually in-creases mechanical stresses because of coefficientof thermal expansion (CTE) mismatches amongvarious materials in the systems, accelerates cor-rosion process, and degrades electrical propertiesfor the dielectrics. High-temperature also reducesmechanical strength, increases creep, or evencauses material phase transition or decompositionin the dielectrics. Additionally, dielectric leakage inelectrical or electronic devices (such as semicon-ductor devices, capacitors, etc.) increases with tem-perature. One of the most important issues in de-signing high-temperature electrics and electronicsis to select high performance dielectrics and struc-


tures to minimize these failure modes. Apparently,conventional polymer dielectrics will lose their di-electric and mechanical properties at such high tem-peratures. Thus, high-temperature resistant polymerdielectrics (HTPDs) with thermal class over H (>180°C) have attracted much attention from both of theacademic and industrial engineering.

As shown in Table 1 and Fig. 1, HTPD materialsusually include fluoro resin, silicone resin, aromaticpolymers, such as polyetheretherketone (PEEK),and heteroaromatic polymers, such as polyimide(PI), polyamideimide (PAI), polyetherimide (PEI),polybenzimidazole (PBI), polybenzoxazole (PBO),polyphenylquinoxaline (PPQ), etc. The commonstructural characteristics for HTPDs include high-energy chemical bonds (C-F, Si-O, etc.) in the mol-ecules, strong inter- and intra-molecular interactionscaused by hydrogen bond (-NH ]n PBI or –CO-NH-in PAI, etc.), or conjugation action along the poly-mer chains (imide ring, imidiazole ring, etc.) [10].These structural features often endow HTPDs withhigh thermal stability, excellent dielectric propertiesas well as good mechanical properties and environ-mental stability. Excellent combined propertiesmake HTPDs good candidates for high-temperatureinsulating applications.

Considerable progress has been achieved in bothof the academic development and commercializa-tion for novel HTPD materials in the past decades.Since the R&D of common fluoro resin, siliconeresin, and aromatic polyamide types of HTPDs hasbeen well reviewed in the literatures[11-13], the stateof art and future developing trends for rarely-reviewedhetero- or aromatic-ring HTPDs, including PI, PAI,PBI, PBO, PEEK, and PPQ in electrical and elec-tronic insulation was introduced in the current work.The molecular design and performance charactersfor HTPD materials were presented. The applica-tions of HTPDs in several important electrical andelectronic fields were also reviewed.

2. HIGH TEMPERATURE RESISTANTPOLYMER DIELECTRICS

2.1. Brief introduction for hightemperature polymers

High temperature polymers (HTPs) are usually char-acterized by their superior physical and chemicalproperty stability at elevated temperatures comparedto common organic polymers [14]. Briefly, the de-velopment of HTPs has experienced three stagesup to now. The first stage is the origin and initialmass-production for HTPs in 1960s, largely driven

by the demands of human space exploration. Vari-ous HTPs, ]nclud]ng, PI, PBI, PBO, and “ladder”polymers were designed and produced. However, alarge proportion of the HTPs gradually died out inthe following years due to poor processability orextremely high cost. The second developing stagefor HTPs is in the range of 1970s to 1980s, with therapid development of electrical and electronic indus-try, especially microelectronic industry. In this stage,for the HTPs originated in 1960s, only those pos-sessing both of high thermal stability, goodprocessability, and relatively low cost survived andachieved commercialization. The third stage is whatwe are experiencing now, that is the functionalizationof HTPs for applications in advanced electrical (high-speed transportation system, high-power energygeneration, storage and transmission, etc.) and elec-tronic (microelectronic and optoelectronic fabrica-tion) applications.

For understanding the applications of HTPs inelectrical and electronic insulation, we can find thetrace from the relationship between the years syn-thetic polymers started being applied to insulationand their highest usable temperatures [15]. Fromthe mid-1960s, HTPs have been increasinglyadopted as dielectrics in insulation applications.This developing trend is closely related with the ur-gent demands of high-temperature and high-reliabil-ity electrics and electronics. Various applicationsof HTPs in electrics and electronics have been re-viewed in the literature during the past decades [16].

Generally, most of the demands on the proper-ties of HTPDs for electrical and electronic applica-tions are similar, including high thermal stability, highdielectric strength, high surface and volume resis-tivity, and so on. However, there still have differentfocuses for these two applications. Thus, when de-signing HTPDs for the specific applications, oneusually addresses different molecular design em-phasis. Fig. 2 summarized various molecular de-sign procedures, including favorable designs andunfavorable ones in developing HTPDs for electricaland electronic applications. The unfavorable proce-dures for these two aspects are usually the same,both of which are in order to preventing the deterio-ration of thermal resistances for the HTPDs. How-ever, the favorite ones are somewhat different. Forinstance, for some specific electrical applications,such as embedded capacitors, they usually demandHTPD materials with a high dielectric constant athigh frequency, a low leakage current, and a rea-sonable high breakdown field [17-20]. Thus, polarand planar substituents, such as nitrile group [21]


and short-chain alkyl groups [22] are favorable inthe design because they can increase the dielec-tric constants of the dielectrics. However, for elec-tronic applications, HTPDs with low dielectric con-stants (low-k) and dissipation factors are highlydesired because they can lower line-to-line noise ininterconnects and alleviate power dissipation issuesby reducing the capacitance between the intercon-nect conductor lines. In addition to providing devicespeed improvements, low-k interlayer dielectrics alsoprovide lower resistance-capacitance delay, mak-ing them superior to low resistivity metal conduc-tors such as copper and silver [23-25]. At this point,HTPDs with apolar and twisted or bulky molecularstructures are preferred. In addition, for microelec-tronic applications, HTPDs with relatively lower cur-ing temperatures (<250 °C) are preferable becausemany kinds of microelectronic components are tem-perature sensitive [26].

2.2. High temperature resistantpolymer dielectrics

As mentioned above, HTPDs have been achievingrapid development both in the basic research andcommercialization in the past decades. Fig. 3 sum-marizes the typical commercially available HTPDs.Their typical chemical structures and commonpreparation procedures are illustrated in Fig. 4 [27].Various forms, including varnish, coating, adhesive,

Fig. 2. Property requirements of electrical and electronic applications for HTPDs.

sealant, film, fiber, plastics, etc. have been devel-oped for HTPDs according to different applications.Generally, liquid HTPD forms are suitable to be usedin wires, cables, and integrated circuits applications;whereas solid ones in printing circuit boards andvarious insulating connectors.

All of these HTPDs possess aromatic orheteroaromatic molecular skeletons in order tomaintain high thermal, dielectric, and dimensionalstability at elevated temperatures. Some easily ther-mal-degradable groups, such as flexible ether link-ages (for PEI, PEEK, etc.) or lateral phenyl sub-stituent (for PPQ) have also to be introduced in or-der to achieve good processability, such as directheat-sealability to copper wires without using anyadditional adhesives. For the rigid HTPDs with in-soluble and infusible characteristics, such as PI andPBO, their easily-processable precursors are oftenused in practical applications [28]. The precursorswill turn into the final HTPDs by curing at elevatedtemperatures. This methodology is widely used inelectrical and electronic insulation applications.

The typical thermal and dielectric properties forthe HTPDs can be clearly understood from the datareported by the supplier or shown in the literatures,which are briefly tabulated in Table 2. Obviously,high glass transition temperatures (T

g), high resis-

tivity, and high dielectric strength are the commonfeatures for the HTPDs. The specific characteris-


Fig. 3. Representative commercially available HTPDs in insulation applications.

Fig. 4. Common preparation procedures for HTPDs.


tics for every HTPD will be briefly introduced andcompared as follows.

2.2.1. Imide polymers (Polyimide,polyamideimide,polyetherimide)

Imide polymers are characterized by the five-mem-bered imide ring in the backbone of the polymers[29]. The physical and chemical characteristics ofthe imide cyclic structure determine the basic prop-erties for the polymers, including favorable high ther-mal and thermal-oxidative stability, high dielectricstrength, high mechanical strength, and unfavorablemoisture sensitive and high water uptakes.(1) Polyimide (PI). PI is one of the most widely usedHTPDs in both of electrical and electronic applica-tions. Meanwhile, PI is the most commercializedspecies in HTPD families. Various PI forms, includ-

* Katpon® HN: poly(pyromellitic dianhydride-co-4,4 -oxydianiline), common PI film Kapton® CR100: corona-resistant PI film; Kapton® MT100: thermally conductive PI films, Dupont, USA; Upilex®-25S: poly(3,3',4,4'-biphenyltetracarboxylic dianhydride-co-1,4-phenylenediamine), high-temperature PI films, Ube, Japan;Vespel®-SP1: unfilled PI engineering plastics, Dupont, USA; Meldin® 7001: unfilled PI engineering plastics,Saint-Gobain, France; Torlon®4203L: unfilled PAI engineering plastics, Solvay, Belgium; Celazole® PBI U-60: unfilled PBI engineering plastics, Celanese, USA; Ultem®1000: common PEI film, Extem® XH1005:high temperature PEI plastics, Sabic, Saudi Arabia; HD-8820: photosensitive PBO varnish, HD-Microsystems,Japan; Aptiv® 1000: common PEEK film, Victrex, England; IP-200®: common PPQ film, IFP-Cemota, France.

Table 2. Typical thermal and dielectric properties for common HTPDs.

HTPDs* Tg(°C) Volume Dielectric Dielectric Dissipation loss

resistivity strength constant factor( cm) (kV/mm)

PI film Kapton® HN >300 1.5×1017 303 (25 um) 3.4 (1 kHz) 0.002 (1 kHz)PI film Kapton® CRC >300 2.3×1016 291 (25 um) 3.9 (1 MHz) 0.003 (1 MHz)PI film Kapton® MT >300 >1016 220 (25 um) 4.2 (1 MHz) Not availablePI film Upilex®-S >350 1015 272 (25 um) 3.5 (1 kHz) 0.001 (1 kHz)PI plastics Vespel®-SP1 >300 1014-1015 22.0 (2 mm) 3.5 (1 MHz) 0.003 (1 MHz)PI plastics Meldin® 7001 >300 1015-1016 22.9 (2 mm) 3.1 (1 MHz) Not availablePAI plastics >300 2.0×1017 23.6 (2 mm) 3.9 (1 MHz) 0.031 (1 MHz)Torlon®4203LPBI plastics 427 2.0×1015 23.0 (2 mm) 3.2 (1 MHz) 0.003 (1 MHz)Celazole® PBIPEI plastics 219 2.1×1017 200 (25 um) 3.2 (1 MHz) 0.005 (1 MHz)Ultem®-1000PEI plastics 267 >1016 25 (1.5 mm) 3.1 (1 MHz) 0.092 (1 MHz)Extem® XH1005PBO varnish HD-8820 300 3.0×1016 470 (3 um) 2.9 (1 MHz) 0.009 (1 MHz)PEEK film Aptiv® 1000 Not Not 190 (50 um) 3.5 (10 MHz) 0.002 (10 MHz)

available availablePPQ film IP200® 365 3.0×1017 - 2.4 (1kHz) 0.0003 (1kHz)

ing PI impregnating varnish (usually PI precursors),PI coatings, PI films, and PI engineering plasticshave been successfully used in insulation industryfor half a century. The typical preparation procedurefor PI is shown in Fig. 4. Generally, PI varnishes orenamels with the form of their soluble precursorspoly(amic acid) (PAA) in polar solvents, such asN,N-dimethylacetamide (DMAc) are widely used forhigh-temperature magnet wire insulation [30]. Dur-ing the high temperature baking, the solvents areevaporated, and at the same time, the PAA precur-sors are dehydrated to facilitate the ring closure withthe elimination of water to afford the final PI coat-ings with a thermal class over 220 °C. This chemi-cal reaction is known as imidization reaction, whichusually needs the high temperature as high as 350°C to be complete. In order to avoid the pin-holesand other defects in the PI coatings, a step-by-stepprocedure is usually adopted for the imidization and


PAA precursors. Some representative PI dielectricsare introduced as follows.

First, general-purpose PI films (such as Kapton®

HN film by Dupont, USA with the chemical struc-ture shown in Fig. 4). These PI films usually providean excellent balance of properties over a wide rangeof temperatures (-269-400 °C). The films with a thick-ness of 25 m show typical dielectric properties ofa thermal class of 220 – 240 °C, dielectric strengtharound 300 kV/ m, volume resistivity over 1017

cm, dielectric constant of 3.2-3.4 at 1 kHz, and dis-sipation factor lower than 0.002 at 1 kHz. In addi-tion, the film has excellent thermal stability with T

g

values higher than 350 °C. Secondly, high tempera-ture resistant PI films (such as Upilex®-S PI film byUbe, Japan). Upilex®-S film is a super-heat resis-tant PI film deriving from biphenyl-type monomers,which is different from that of Kapton® HN (Fig. 4).This formulation makes the PI film exhibit outstand-ing thermal and thermo-oxidative stability, dimen-sional stability, excellent dielectric properties, lowwater absorption and very high chemical resistance.Tsukiji and coworkers evaluated the thermal degra-dation of endurance of Kapton® HN and Upilex®-Sfilms in severe environments [31]. The experimentalresults revealed that both of the two PI films wereable to be used in electrical insulation systemsoperating at temperatures up to 350 °C for 20000hours in inert gas. In air, only Upilex®-S was able tobe used at 300 °C. The diphenylether moiety inKapton® HN film was thought to be vulnerable tothermal aging in air. For insulating applications,Upilex®-S film shows excellent electrical character-istics (thermal class: 250 °C; dielectric strength:272 V/ m for 25 m film and 440 kV/ m for 75 mfilm at both of 25 °C and 200 °C) over a wide rangeof temperatures and frequencies. Even at high tem-peratures, the film shows almost no deterioration inits electrical properties. Diaham and coworkers in-vestigated the dependence of dielectric breakdownof Upilex®-S film on the area, thickness, and tem-perature [32]. In the test temperature range of 25-400 °C under direct circuit (DC) ramps, no tempera-ture-dependence of the dielectric strength was ob-served. The dielectric breakdown field of the filmremained relatively high (>2 MV/cm) up to 400 °C.Upilex®-S film also exhibits a low level of insulatingdefects, making it an optimal choice for electricaland electronic applications that demand high reli-ability. The other features for the film include smallvalues for both heat shrinkage and thermal linearexpansion coefficients (CTE: 12 ppm/°C from 50-200 °C) and high flame retardancy (UL94 VTM-0).These features make the film ideal for use in high-

temperature electronic insulating application. Thirdly,corona-resistant PI films (such as Kapton® CRC filmby Dupont, USA). These films are developed spe-cifically to withstand the damaging effects of co-rona, which can cause ionization and eventual break-down of insulating dielectrics when the voltage stressreaches a critical level. Kapton® CRC PI film showscorona resistance or voltage endurance greater than100,000 h at 20 kV/mm at 50 Hz. The film alsoprovides twice the thermal conductivity (0.385 W/mK) of standard PI film. These combined propertiesmake Kapton® CRC film good candidate for appli-cations in the insulation of transformers, tractionmotors, frequency conversion motor and electricalrotating machines. Fourthly, thermal conductive PIfilms (such as Kapton® MT film by Dupont, USA,etc.). The PI film usually offers an excellent combi-nation of electrical properties, thermal conductivityand mechanical properties, making it ideal for usein controlling and managing heat in electronic insu-lating applications, such as printed circuit boards,etc. Fifthly, PI engineering plastics (such as Vespel®

SP1 by Dupont, USA; Meldin® 7001 by Saint-Gobain, France). The PI engineering plastics areusually processed into various parts and shapes forelectrical and electronic insulation applications. Theparts are usually more ductile than ceramics andlight than metals. They have outstanding dielectricproperties over wide temperature (25-250 °C) andfrequency (102-105 Hz) ranges. The combination ofgood dielectric properties, high strength and excel-lent thermal and radiation resistance makes the PIplastics outstanding candidates for insulations insevere environments.

Although PIs have found various applications inelectrical and electronic insulation, the insoluble andinfusible nature limited their applications. For ex-ample, in magnet wire insulation, PI films have tobe combined with heat-fusible fluoropolymer film,such as fluorinated ethylene propylene (FEP) filmin order to achieve hot windings. This methodologyhas been well established for various PI dielectrics,such as heat-sealable common PI films (Kapton®

FN), heat-sealable corona resistant PI films (Kapton®

FCRC), and so on. Alternatively, other imide poly-mers with enhanced thermo-processability, such asPAI and PEI have also been developed.(2) Polyamideimide (PAI). PAI combines the goodproperty merits of PI and wholly aromatic polyamide.Thus, the polymers usually possess good thermalstability, excellent dielectric and mechanical prop-erties, good processability, and chemical resis-tance. PAI can usually be used with two formats -varnish and plastics in insulation industry. As shown


in Fig. 4., from the aspect of property or cost, thePAI resin used most often for enameled wires canbe obtained mainly by a synthesis reaction of 4,42-diphenylmethane diisocyanate (MDI) and trimelliticanhydride (TMA) in a polar, aprotic solvent such asN-methyl-2-pyrrolidinone (NMP) at high tempera-tures. The obtained PAI resins are soluble in sol-vents with the fully cyclized forms to afford the PAIenamels [33].

PAI varnish are widely used as class 220 °Cenamels and applied mostly as overcoat to producedual coating wires using for hermetic motors andFreon-resistant wires [34]. In addition, with the tre-mendous speed increase for winding of motors andother electrical devices, PAI have been widely usedas magnet wire coatings due to the good abrasionresistance and self-lubricating feature in order tomaintain low defect rates for the products. Typically,in electrical insulations, the wire enamel structureof polyesterimide (PEsI) layer underneath and PAIon top of it is the most common magnetic wiresused in low-voltage machines operated under hightemperature. This insulated magnetic wire has verygood mechanical properties, making the windingprocess easier because the wire can be stretchedand bent roughly. PAI engineering plastic (such asTorlon® 4203L by Solvay, Belgium, etc.) has highstrength and stiffness up to 260 °C and can be pro-cessed into various insulating parts, even with com-plicated structures via the low-cost injection mold-ing procedure. This characteristic greatly expandsthe applications of PAI in insulation industry.

(3) Polyetherimide (PEI). PEI represents a modi-fied PI with flexible ether linkages in the repeatingunit of the polymer. A typical of preparation proce-dure for PEI is shown in Fig. 4. Due to its high solu-bility in solvents, PEI can usually be prepared bythe one-step high-temperature polycondensationprocedure using bisphenol A dianhydride (BPADA)and m-phenylenediamine as the starting monomers.Although introduction of ether groups inevitably sac-rifices the thermal stability of the pristine PI, goodprocessability, such as heat sealability is usuallyachieved. PEI dielectrics are heat sealable to a va-riety of materials, including metal (copper foils, etc.)and other polymers (PI, etc.) due to their thermo-plastic nature; thus can be wound to the wires with-out using any additional adhesives. In addition, PEIexhibits good resistance to ultraviolet radiation andgamma radiation without stabilizers. Thus it mightbe used in nuclear radiation environments for longterms[35].

PEI can be available with the forms of film andengineering plastics. The first generation of PEI,

such as Ultem® 1000 (Sabic, Saudi Arabia) showsa T

g value of 219 °C, which can usually meets the

class H electrical insulation requirements for motorand transformer applications. The good design flex-ibility for PEI polymers has now offered the secondgeneration of high-T

g type alternative PEIs, such as

Extem® XH1005 (Tg: 267 °C by Sabic, Saudi Arabia),

which can meet the demands of class 200 insula-tion. Very recently, PEI resin (Ultem® 9085, Sabic,Saudi Arabia) which can be processed into variousinsulating parts by 3D printing technique has alsobeen reported [36].

2.2.2. Polybenzimidazole (PBI) andpolybenzoxazole (PBO)

PBI and PBO are initially commercialized as high-performance fibers due to their high strength, highmodulus, and excellent flame retardancy [37]. Asillustrated in Fig. 4, conventional PBI and PBO poly-mers are usually prepared via high-temperature poly-condensation procedures so as to achieve high de-gree of cyclization. Lately, it was discovered thatthese polymers have excellent properties as dielec-trics for insulation applications. Thus, various insu-lating varnishes, coatings, enamels and plasticsbased on these polymers have been developed. Forexample, PBI plastics (such as Celazole® PBI U-60 by PBI Performance Products, USA) are knownas the highest thermal stable thermoplastics with aT

g higher than 400 °C. Electrically insulating parts

made from PBI have fine electrical properties, goodchemical stability, flame retardancy, and excellentresistance to heat, cold, radiation, and other severeenvironments. Recently, PBI coatings (such as S10and S26 solution by PBI Performance Products,USA) with solid contents higher than 10 wt.% inpolar solvent achieve commercialization, which aresuitable for film casting, dip coating, spray coatingand resin impregnation. The cured coating has thesimilar properties with PBI matrix. These productshave great potential applications in high-tempera-ture magnet wire insulation.

PBO has the advantage over other high-tempera-ture polymer films due to the absence of polar groupsin its repeating unit. The absence of polar groups inthe polymer prevents the formation of hydrogen bondbetween the polymer and water, therefore resultingin less water absorption in the film and low dielec-tric constant (<3.0 at 1MHz) and dielectric loss fac-tor. Additionally, PBO is known for its excellent ther-mal stability and thermo-oxidative resistance, highelectrical resistivity, and high dielectric strength thatremains stable over a long temperature period. Re-


cently, a key technique achieving fully cyclizationof PBO at temperatures lower than 250 °C has beenachieved [38]. This curing temperature is much lowerthan that of common PI dielectrics which have to becured at 350 °C or higher. A series of low-tempera-ture curable PBO dielectrics have been commer-cialized recently by HD-Microsystems, Japan, in-cluding HD-8921 with curing condition of 250 °C for1 h; HD-8930 of 225 °C for 1 h, and HD-8940 of 200°C for 1 h. This feature is critical for most micro-electronic and optoelectronic applications, in whichmany components are high temperature sensitive.The good combined properties make PBO goodcandidates for in microelectronic packaging.

2.2.3. Polyphenylquinoxaline (PPQ)

PPQ represents a class of high performance ther-moplastic heteroaromatic polymers, which is char-acterized by the phenyl-substituted quinoxaline ringin the structure [39]. The special molecular struc-ture endows the polymers many favorable proper-ties, such as high thermal and thermo-oxidative sta-bility; good solubility in organic solvents and rela-tively low dielectric constants; and excellent hydro-lytic resistance. Thus, PPQs have found variouspotential applications as dielectrics for electrical andelectronic fabrication [40]. Although PPQs havebeen extensively studied for half a century, they havenot been achieved wide commercialization like theother heteroaromatic polymers, such as PI. This ismainly due to the relatively high cost and very lim-ited commercial availability of the starting mono-mers, including bis( -diketone) and bis(o-diamine)compounds shown in Fig. 4. Nevertheless, PPQ stillattracts the attention in dielectric materials R&Dbecause the polymer has been proven to be one ofthe best hydrolytic resistant high temperature poly-mers and possesses excellent hydrolysis resistanceto various (neutral, acidic or alkaline) waters not onlyat room temperature, but at elevated temperatures.PPQ films could maintain integral shapes and flex-ibility even after boiling in distilled water at 250 °C(vapor pressure of water: 3.9 MPa) for 60 h. Underthe same conditions, the PI films have been com-pletely hydrolyzed. Commercially available IP200®

PPQ films are not degraded when aged for one monthin 50% sodium hydroxide solution up to 150 °C or40% sodium hydroxide solution up to 160 °C. Theyare also stable in boiling water up to one year. Thisunique characteristic makes PPQ good candidatesin various insulation applications with high humidityand high temperature circumstances. In addition,the bulky phenyl-substituted quinoxaline rings in

PPQs is nonpolar, so the dielectric constants ofPPQs are usually lower than those of other hightemperature polymers. This feature makes PPQs agood low dielectric constant (low-k) material as theinterlayer dielectric (ILD) for ultra-large integratedcircuit (ULSI) fabrications [41]. Recently, Liu andcoworkers developed a low-cost route for PPQs.Various functional PPQs with relatively low cost whilemaintaining intrinsic thermal stability, dielectric prop-erties, and hydrophobic nature have been developed[42-45]. This endeavor greatly expands the wideapplication of PPQ in insulating industry.

2.2.4. Polyetheretherketone (PEEK)

PEEKs are a family of polymers containing alterna-tive ether and ketone groups in their repeating units,whose typical synthesis procedure is shown in Fig.4. Such kind of chemical structure imparts PEEKssemi-crystalline nature with high thermal resistanceand excellent dielectric properties. In addition, un-like the other heteroaromatic polymers mentionedabove, PEEK can usually be winded with copper oraluminum wires directly at elevated temperaturewithout using any adhesives. This is quite benefi-cial for improving the reliability of the insulating prod-ucts. At last, PEEK usually possesses very stabledielectric properties over a wide range of tempera-tures, frequencies and humidity. Especially, the goodresistance to steam makes PEEK unique dielec-tric for insulating applications because steam is avery harsh environment for insulating materials. It isa combination of water and high temperature andeasily causes the hydrolysis of common HTPDs. Ithas been proven that PEEK can resist hydrolysisin temperatures up to 250 °C [46].

PEEK can be used as HTPDs with both formsof engineering plastics and film. The recently com-mercialized PEEK film, Aptiv® 1000 (Victrex, En-gland) has a semi-crystalline molecular structure,exhibiting superior moisture, chemical, abrasion, andradiation resistance to other polymer dielectric films.The melt-processable PEEK film has a thermal in-dex of 220 °C.

3. ENGINEERING APPLICATIONS OFHTPDS

3.1. Electrical insulation

3.1.1. High-temperature magnet wireinsulation

Enamel-insulated copper and aluminum magnetwires are used principally in the electrical and elec-


tronics industries for coils, inductors, transformers,generators, armatures, solenoids, and other wind-ings [47]. Wire may be insulated and protected witha wide variety of coating materials which may beclassified according to the manner of application.For example, wires may be dipped in a liquid bathof the resin solution (known as varnish, enamel orimpregnating coating, etc.) or coated by extrusionof solid thermoplastic resins. For HTPDs, they canusually be applied to the wires either by dip coatingor by extruded coating, although the former manneris used more frequently.

As mentioned above, magnet wire may be clas-sified into several ratings according to NEMA stan-dard (Table 1). For high temperature magnet wireswith the maximum serving temperature over 200 °C,thermal class is clearly the most important param-eter to be considered when designing the dielec-trics. For instance, Petitgas, et al. investigated therelationships between chemical structure and elec-trical behavior in the high temperature aging of cop-per wires enameled with various HTPDs, includingcrosslinked polyesterimide (PEsI) with a thermalclass of 180-200 °C, PAI with a thermal class of220 °C, and PI (PAA precursors) with a thermal classof 240 °C[48]. Copper wires were enameled by PEsI(thickness: 0.025 mm), PEsI+PAI (thickness: 0.035mm), PAI (thickness: 0.03 mm), and PI (thickness:0.035 mm), respectively. All the wire samples werethermally aging in the temperature range of 200-400 °C in an oven. The experimental results revealedthat the enameled wires based on PEsI started todegrade above 300 °C, whereas those based on PAIand PI were stable before 400 °C. Meanwhile, PAIpresented promising performances for high tempera-ture rotating machine applications compared to thoseof PI. Schadler and coworkers investigated the hightemperature breakdown strength and voltage endur-ance of nanofilled PAI dielectrics [49]. In the experi-ments, nanoscale silica and alumina were filled intoPAI, respectively, for high temperature wire enamelapplications. Experimental results showed that, atfiller loading of 5-7.5 wt.%, the direct circuit (DC)breakdown strength improved for both the PAI sys-tems at 30 °C and 300 °C; however, only the alu-mina system exhibited an increase in alternativecircuit (AC) breakdown strength. The authors as-cribed this phenomenon to the electron scatteringand corona resistance of the alumina nanoparticles.This result is quite beneficial for designing HTPDswith improved dielectric strength. Recently, research-ers from Kennedy Space Center (KSC), NationalAeronautics and Space Administration (NASA) re-ported a PI wire insulation repair system [50]. In the

technique, thin film PI patches are applied to thedamage areas of wire insulation with a heating de-vice that adhering the PI repair film into the place.The wire repairs made with the system are perma-nent, flexible and much less intrusive than repairsmade using current techniques and materials. Thistechnology is well suited for all applications of PIand other high temperature polymer enameled wireconstructions and might find potential applicationsin aerospace wiring, automotive wiring, and otherindustrial fields. The used patches are low-tempera-ture meltable PIs derived from bisphenol Adianhydride and aliphatic or aromatic diamines anddouble bonds-containing crosslinker. The cured PIpatches have good electrical properties.

For industrial applications, various magnetic wiresenameled with HTPDs have been achieving wideapplications in high-tech areas. For instance, PIinsulated wires and cables, such as ESCC 3901001 developed by Axon2 Cable&Interconnect,France with the operating temperature of -100~200°C have been successfully applied in the electricalsystems for various space projects, including LISA(Laser Interferometer Space Antenna) Pathfinder forlow-frequency gravitational wave detection [51],Eurostar 3000 satellite platform [52], and so on. Thesimilar wires and cables insulated with PI/fluorothermoplastic (ESCC 3901 018) with the op-erating temperature of -200~200 °C have been ap-plied in the electrical system of GOCE (gravity fieldand steady-state ocean circulation explorer) space-craft operating in low orbit orbits [53]. Excellent ther-mal, dielectric and radiation stability for the PI coat-ings enable them good insulating protection for thespacecrafts.

3.1.2. Aerospace and aviationelectrical machines insulation

HTPDs have been evaluated as dielectric films foraviation and aerospace electrical insulation for sev-eral decades. For example, PBI, together with poly-P-xylene (PPX) and Teflon® perfluoroalkoxy (PFA)were characterized to explore their possible use asa replacement for PI (Kapton®) in aerospace highvoltage wiring insulation[54]. Although PI (Kapton®)has been widely used for wire insulation and energystorage for aerospace and space power systems,some performance defects including arc-trackingand cracking under a combination of high tempera-ture and humidity limited its reliability. The evalua-tion results indicated that PBI exhibited excellentthermal stability and dielectric strength in thetemperature region of 250-300 °C as compared to


Fig. 5. Aircraft electrical power generating capacityby aircraft type.

Fig. 6. Potential applications of PI dielectrics in MEAinsulation.Kapton® film. This makes it more useful for high volt-

age and high temperature aerospace wiring insula-tion applications. PPQ has also ever been evalu-ated as thermally resistant aerospace wire insula-tion in place of PI due to its superior toughness,thermal stability, low density (15% lower than thatof PI), and especially hydrolytic stability [55].

For aviation application, recently, with the rapiddevelopment of aircrafts towards high-speed, light-weight, high reliability, and long servicing life, moresevere properties are required to the electrical ma-chines insulating materials. For example, the newlydeveloped civil and military aviation motors havelarger power requirements, higher power output,smaller volume, and lighter weight. Fig. 5 showsthe development of power generation capacity forcommercial aircrafts [56]. The increasing currentdensity inevitably increases the working tempera-ture for the electrical motors. Some aviation electri-cal machine can reach a long term servicing tem-perature of 250 °C, short term of 290 °C, and in-stantaneous temperature over 400 °C. Such a high-temperature environment requires the dielectricspossessing excellent thermal and dielectric stabil-ity.

Abdelhafez reviewed the development status ofmore electric aircrafts (MEAs) and their electricpower generation systems [57,58]. MEAs (such asAirbus 380 and Boeing 787) achieve numerous ad-vantages over conventional fuel-powered aircrafts,including improving the aircraft performance, de-creasing operating and maintenance costs, and re-ducing the air pollutant gases [59]. However, MEAsput great pressure on the electrical system eitherin the amount of the required power and the pro-cessing and management of the power. Electricalinsulation is a big concern due to the high power

generation level. To make MEA the preferred op-tion, the development of new dielectrics for compo-nents is necessary. For insulation materials, higheroperating temperatures (up to or above 300 °C) areusually required. Upilex®-S PI film and Eymyd® PIfilm (a fluorinated PI film) are thought to be promis-ing insulating dielectrics for MEAs electrical insula-tion system due to their good thermal stability andgood resistance to humidity, ultraviolet radiation,basic solution and solvent at high altitudes [60,61],as illustrated in Fig. 6.

3.1.3. High-speed train electricalmachines insulation

Wide application of adjustable-frequency and vari-able-speed techniques in high-speed train greatlypromote the development of advanced HTPDs dueto the specific property demands to the dielectricsfor the application [62]. Generally, for the dielectricsusing in high-speed train electrical machines insu-lation, excellent corona resistance, together withstable dielectric properties at high operating tem-peratures are highly desired. Conventional pure poly-mer dielectrics, such as Kapton® HN used for ordi-nary train electrical motor insulation usually cannotendure the corona damage and might lose the insu-lation ability under long-term electrical stress. Incontrast, PI films with corona-resistant characteris-tics, such as Kapton® FCRC has been adopted inthe European rail industry to improve the efficiencyand durability of AC traction motors on high speedlocomotives (such as Eurostar, etc.), where it sig-nificantly outperformed traditional insulation sys-tems.


China is upgrading conventional railway lines andconstructing tens of thousands of kilometers of high-speed railway lines. The total length of high-speedpassenger lines will surpass 18,000 km within thenext ten years, almost half of which will be 350 km/h lines. In the electrical machines, such as tractiontransformers for China Railway High-speed (CRH)electric multiple unit (EMU), common PI tapes, filmsand corona-resistant PI films (Kapton® FCRC) havebeen adopted as insulating materials [63]. In thisarea, numerous researches have been performed.For instance, Yang et al. investigated the coronaaging in Kapton® HN PI film under alternative circuit(AC) voltage by Fourier transform infrared spectros-copy (FTIR) and the dielectric measurement [64]. Itwas found that the ether linkage, some of C-H bondsin aromatic rings and C-N-C bonds in imide werefirstly broken by corona discharge. Then, with theincrease of aging time, the cleavage of the imidegroup including C-N-C and C=O bonds occurs. Atthe same time, the oxidative degradation producedcompounds such as carboxylic acid, ketones, andaldehydes. It has been well established that intro-duction of specific nanoparticles (alumina, titania,etc.) into the dielectrics can efficiently improved thecorona resistance of the obtained nanodielectrics.This improvement is mainly attributed to the elimi-nation of partial discharge, space charge, ioniza-tion, and other dielectric damage caused by thecorona phenomenon. Zhou and coworkers studiedand compared the charge transport mechanism instandard Kapton® HN film and corona-resistantKapton® CRC film at a thickness of 0.025 mm[65,66]. The experiments indicated that the dielec-tric constant of Kapton® CR film is larger than that

Fig. 7. Applications of HTPDs in microelectronic fabrications.

of Kapton HN, which enhances the barrier height forinjecting charge into the dielectric, thereby increas-ing space charge accumulation threshold field. Inaddition, introduction of nano-particles raises thenumber of traps in Kapton® CR dielectric, therebyforming stable space charge electric field, whicheffectively increases dielectric properties of the film.Akram and coworkers prepared the PI/Al

2O

3

nanocomposite film by in-situ polymerization tech-nique, together with the physical mixing (stir andultrasonic) of nano-Al

2O

3 particle, followed by high-

temperature imidization [67]. The degradationmechanism of nano-Al

2O

3 filled PI film due to sur-

face discharge under square impulse voltage wasthen investigated. The results showed that failure ofspecimens is mainly caused by surface dischargewhich could be suppressed by the addition of nanoparticles. The nano-Al

2O

3 fillers improved the corona

resistant property of PI film significantly. The co-rona resistant lifetime of PI/Al

2O

3 sample is six times

longer than that of pure PI. Liu, et al. studied theeffects of nano-Al

2O

3 contents on the dielectric prop-

erties of the PI/Al2O

3 composite films[68]. They pre-

pared the nanocomposite film by sol-gel method withaluminum isopropoxide as the precursor of nano-alumina in order to achieve a homogeneous disper-sion of the nano particle. Experimental results re-vealed that addition of Al

2O

3 nanoparticles greatly

improved the corona resistance of PI compositefilms. When the Al

2O

3 content was 12 wt.%, the

corona resistance time reached 136 hours, whilethe pure PI film was only 0.5 hour. This improve-ment in corona resistance of PI/Al

2O

3 composite

films should be attributed to the preventing chargeaccumulation effect of Al

2O

3 nanoparticles. Fan, et


al. studied the impact of air relative humidity on thecorona-resistant property of Kapton® CRC film [69].It was found that the corona resistant aging for thePI film decreased logarithmically with the increaseof the relative humidity. This phenomenon is closelyrelative with the intrinsically hydrophilic nature ofthe PI film.

3.2. Electronic insulation

Electronic industry, especially microelectronic andoptoelectronic industry sees a tremendous devel-opment in the past decades. HTPDs materials, espe-cially PI and PBO have good heat resistance, me-chanical properties and electrical insulation; thushave been widely used in microelectronic and opto-electronic industry [70,71]. The typical applicationsof HTPDs in microelectronics are illustrated inFig. 7.

PI and PBO chemistries deliver unique proper-ties as interlayer dielectrics, redistribution layers,alpha-particle shielding layers, solder bump stressbuffers, and passivation layers for microelectronicapplications, including integrated circuit packaging,

Fig. 8. Simplification of microelectronic patterning with photosensitive HTPDs. Reprinted with permissionfrom T. Higashihara, Y. Saito, K. Mizoguchi and M. Ueda // Reactive Functional Polymers 73 (2013) 303, ©2013 Elsevier.

multi-chip modulus (MCM), microelectromechainicalsystems (MEMS), monolithic microwave integratedcircuits (MMIC), etc. The polymers provide uniqueinsulating protection and other functions to the elec-tronic devices. More importantly, both PI and PBOchemistries offer flexible formulations of positive ornegative photosensitive (or photodefinable) versions,which greatly simplify the microelectronic pattern-ing, through-hole and other fabrication procedureswithout the aid of conventional photoresists, asshown in Fig. 8 [72]. Thereby, photosensitive HTPDscan obviously minimize the required facilities andrealize a highly efficient productivity and low costfor microelectronic manufacturing. HTPDs with pho-tosensitivity usually are double-purpose polymerdielectrics and are used as permanent materials oras temporary processing aids.

PI was introduced into electronics in early 1980sdue to its suitable combination of high thermal sta-bility, good mechanical properties, and a compara-tively low dielectric constant. Lately, the rapid de-velopment of microelectronic industry desires dielec-trics with lower dielectric constant, low moisturesensitivity, higher thermal and thermal-oxidative sta-


bility so as to achieve much rapid signal transmis-sion speed. Then, PBO dielectrics attract muchmore attentions in recent years. Ueda and cowork-ers summarized the R&D progress for photosensi-tive PI (PSPI) and photosensitive PBO (PSPBO),respectively [73-75]. From these reviews, we canclearly find the developing course of PI and PBOdielectrics in microelectronic applications.

Very recently, PSPBOs with low curing tempera-tures have been paid much attention as dielectricsfor microelectronic integration due to their good com-patibility with modern microelectronic fabricatingprocess, good environmental compatibility using anaqueous alkaline solution as a developer, togetherwith their intrinsic excellent electrical, thermal andmechanical properties. Conventional PBO dielec-trics are usually formed through cyclization ofsoluble PBO precursors, namely poly(o-hydroxyamide)s (PHAs) by thermal treatment over300 °C. This high-temperature process is hardlyapplicable to some high-temperature sensitive inte-grated circuit devices, such as MEMS. Recently,the unique technique for low-temperature curing ofPHAs is successfully overcome by modifying thePBO molecular structure and using the cyclizationpromoter, which greatly expands the wide applica-tion of PSPBO dielectrics [76]. Roberts comparedthe applications of high-temperature and low-tem-perature curable PI and PBO dielectrics in wafer-level chip scale packaging (WLCSP) [77]. WLCSPis thought to be the key technique achieving smallerand reliable mobile devices, such as smartphone,camera, tablet, etc. [78]. PI and PBO materials arerecognized as the ideal dielectrics of choice forWLCSP fabrication due to their excellent thermaland chemical stability, relatively low dielectric con-stant and stress levels, and excellent mechanicalproperties. In addition, the commercialization ofphotodefinable versions for PI and PBO has enabledWLCSP by reducing the number of steps and en-hancing the reliability of the final devices. SeveralHTPDs commercially available from HD-Microsystems, Japan including high-temperaturecurable PI (HD-4100, cure condition: 350-390 °C/1h) and PBO (HD-8820, cure condition: 300-350 °C/1h); and low-temperature curable PBO (HD-8930 andHD-8940) were compared in the investigation. It wasfound that the low-temperature curable PBO dielec-trics could provide comparable dielectric strength,adhesion strength, mechanical properties, moistureabsorption, and volume resistivity with the high-tem-perature counterparts. However, the WLCSP waferwarpage due to the thermal stress caused by tem-perature cycling for the low-temperature PBOs were

much lower than those of the high-temperature onesdue to the relatively lower curing temperatures lessthan 250 °C. Very recently, Nishimura and cowork-ers reported the 200 °C curable positive-tonephotodefinable PBO (HD-8940, cure condition: 200°C/1h) and evaluated its application for fan-out wa-fer level package (FOWLP) [79]. FOWLP technol-ogy is an enhancement of standard WLCSP devel-oped to provide a solution for semiconductor de-vices requiring a higher degree of integration and agreater number of external contacts [80]. It providesa smaller package footprint with higher input/output(I/O) along with improved thermal and electrical per-formance. In FOWLP, it is preferable that the curingtemperature of the buffer coatings and interlayer di-electrics is less than 300 °C for reducing the ther-mal damage of semiconductor devices and materi-als. In the evaluation, the PBO was cured at 200 °Cfor 1 h in nitrogen atmosphere. The experimentalresults indicated that the cure PSPBO exhibitedexcellent reliability against various reliability tests,including temperature cycle test (-55-125 °C, 500cycles), temperature humidity test (85 °C/85% RHfor 168 h) followed by 3 times reflow test (260 °C/10s). Thus, it might find various applications inFOWLP.

Photosensitive PI with ceramic nanoparticles fill-ers have also been widely studied for microelec-tronic applications [81]. The addition of nanosizedceramic fillers to a photoresist is driven by the fol-lowing aspired aspects: improved sensitivity to elec-tromagnetic radiation of a certain wavelength region;improved resolution; improved chemical resist sta-bility; improved mechanical stability during process-ing; tailoring of the coefficient of thermal expansion;and introduction of new functionalities like electri-cal conductivity or magnetic properties. For example,surface modified nanosized SiO

2 (10–50 nm par-

ticle size) has been reported to be beneficial to im-proving the process stability of a photosensitive flu-orinated polyimide due to the enhanced interactionof the particle large surface area and the polymermatrix [82].

4. CONCLUSIONS AND FUTUREPROSPECTS

As a class of high performance polymer dielectrics,HTPDs have been increasingly used in advancedelectrical and electronic applications although theyusually have much higher price than those of com-mon polymer dielectrics. Thus, up to now, HTPDsare mainly used in special applications where theirhigh cost is compensated by their special charac-


teristics. Nevertheless, HTPDs are attracting moreattention and becoming indispensable for high-per-formance insulation. The foreseeable R&D trendsfor future HTPDs might include the following aspects.(1) Theoretically design and numerical simulationfor HTPDs. Molecular design and property predic-tion via computational methods is undoubtedly at-tractive before large-scale production for HTPDs dueto their high cost. Computational strategies forHTPDs design should be further explored and es-tablished [83].(2) Low-cost preparation and manufacture proce-dures for HTPDs. Cost-effective synthesis and manu-facturing procedures for HTPDs is of great concernfor their wide applications in advanced insulation.(3) Property modifications, functionalization and newmaterials R&D. Modification of the performancedefects for the current used HTPDs is undoubtedlybeneficial for expanding their applications. The top-ics include decreasing curing temperatures, improv-ing moisture resistance, improving environmentalcompatibility, and so on. In addition, HTPDs withspecial function, such as super-lightweight, flexibleand foldable, etc. are always highly desired in ad-vanced insulation applications. For example, veryrecently, flexible and bendable high-temperature di-electric materials which can work stable at 250 °Chave been reported [84].

In summary, HTPDs science and technology isan interdiscipline of polymer chemistry, material,electrotechnics, electronics and other related sub-jects; thus their development is highly dependenton the cooperation of scientists and engineers fromdifferent areas. With the rapid development of ad-vanced electrical and electronic industry, HTPDs willsee a bright future.

ACKNOWLEDGEMENT

Financial support from the Fundamental ResearchFunds of Beijing Jiaotong University (E14JB00130),National Basic Research Program (973 Program)of China (2014CB643605), and National NaturalScience Foundation of China (51173188) are grate-fully acknowledged.

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