1. Introduction

Epoxy resins (EP) show advantages in the ease of operating, low shrinkage upon curing, superior adhesion, outstanding thermal resistance, good electrical insulation, and fine mechanical performance [1–4]. Thus, the resin has been widely applied in electronic fields, including conductive adhesives, conformal coatings, printed circuit boards, and flip-chip encapsulation, etc. [5,6]. Among them, cycloaliphatic epoxides, featuring in the aliphatic backbone and fully saturated structure, have attracted much attention for years and have become a good choice in high-frequency electronic devices requiring low dielectric constant and dielectric loss [7–10]. Usually, cycloaliphatic epoxides are synthesized by the peroxidation of aliphatic alkenes, rather than through the condensation of bisphenol A with epichlorohydrin. They are free of chloride and are often cured with anhydrides. As a result, cycloaliphatic epoxy resins outperform the widely used diglycidyl ether of bisphenol A (DGEBA)-type resins in the dielectric performance [11,12]. In addition, cycloaliphatic EP present good weather and UV resistance, owing to the absence of aromatic groups in the molecular structure [13,14]. For this, they can be very durable as indoor and outdoor power equipment in indoor and outdoor conditions; e.g., transformers, HV generators, motors, and switchgear. However, cycloaliphatic epoxy materials are very easy to catch fire and they then burn violently, accompanied by dripping due to their relatively high contents of aliphatic and cycloaliphatic segments [15,16].

retardants, gradually become estranged from the mainstream of electronic fields, due to the consequent environmental problems, human health issues, and recycling requirements of electric equipment [17,18]. For this, halogen-free flame retardants, especially phosphoruscontaining ones, have been valued for their chemical versatility, environmental friendliness, and high flame-retardant activities in both the gaseous and condensed phases [19,20]. In addition, intrinsic flame-retardant epoxy materials are more highly welcomed than other methods and approaches (e.g., additives and coatings) in the field, as they often exhibit high flame retardancy with well-balanced performance and environmental tolerance [21,22]. Using functional epoxy monomers or curing agents, intrinsic flame-retardant resins can be achieved by incorporating flame-retardant elements and groups into the macromolecular chain [23,24]. Nonetheless, concerning cycloaliphatic epoxy systems, some phosphorus-containing epoxy monomers have been prepared in the past few years, while developing the modified anhydrides as the flame-retardant curing agent is rarely reported. Wang et al. [25,26] developed a series of phosphorus-containing cycloaliphatic epoxy monomers, aiming for the recycling of the integrated circuit without damaging the circuit board under heating. Interestingly, the cured resins rapidly decomposed and then completely lost their strength under relatively low temperatures, lying within the desired temperature range for reworking operation. However, the obtained resins showed unsatisfactory flame retardancy and had a relatively low limiting oxygen index value (not higher than 23.9%). At present, it is still a big challenge for producing halogen-free flame-retardant cycloaliphatic epoxy materials.

Aiming for obtaining flame-retardant epoxy systems with a good insulation performance for electronic packaging materials, in this work, we synthesized a kind of phosphorus-containing difunctional cycloaliphatic epoxide (called BCEP) through the epoxidation of phosphorus-containing olefin (BCP), using diphenyl phosphoryl chloride and 3-cyclohexene-1-methanol as the precursor. Then, triglycidyl isocyanurate (TGIC) was further mixed with BCEP in different proportions to achieve epoxy systems that are rich in phosphorus and nitrogen elements, which was then cured with 4-methylhexahydrobenzene anhydride (MeHHPA) to obtain a series of intrinsic flame-retardant epoxy materials. In the system, TGIC was considered to improve the flame retardancy of the resin through the so-called phosphorus-nitrogen synergism effect. The curing kinetics of the epoxy system was investigated using the non-isothermal procedure of differential scanning calorimetry (DSC) at different heating rates. Furthermore, the flame retardancy and burning behaviors of the cured resin were measured using the limiting oxygen index (LOI) and cone calorimeter test (CCT). The thermal stability of the cured resin was studied via thermogravimetric analysis (TG). The frequency dependence of the dielectric constant, and dielectric loss for the materials was determined using a broadband dielectric spectrometer. In addition, the chemical degradation behavior of the cured resin under mild conditions was studied. In addition, a kind of commercially available cycloaliphatic epoxides (named ERL-4221) cured in the same condition was applied as the reference sample to compare with the obtained

2. Results and Discussion

The curing kinetics of the epoxy system were studied using the DSC non–isothermal procedure at heating rates of 5, 10, 15, and 20 C min 1. Figure 2a–c show the curves of procedure at heating rates of 5, 10, 15, and 20 °C min−1. Figure 2a, 2b, and 2c show the hcueravteflso wof vhaeluatesflvoewrsuvsatleumespevreartsuursesteomfBpCerEaPtu1-rTeGsIoCf3,BBCCEEPP1–1T-TGGICIC3,1,BaCnEdPB1C–TEGPI3C-T1,GaICnd1, all of which present a single peak with a different exothermic peak temperature (Tp). This BCEP3–TGIC1, all of which present a single peak with a different exothermic peak temperindicates that BCEP-TGIC systems show a relatively high curing activity and they can be ature (Tp). This indicates that BCEP–TGIC systems show a relatively high curing activity efficiently cured under moderate conditions. Then, curing kinetic parameters are listed and they can be efficiently cured under moderate conditions. Then, curing kinetic paramin Table 1, including activation energy (Ea) and the Arrhenius pre-exponential factor (A), eters are listed in Table 1, including activation energy (Eα) and the Arrhenius pre–expoevaluated using the Kissinger equation and the Ozawa equation [29,30], where b represents nential factor (A), evaluated using the Kissinger equation and the Ozawa equation [29,30], the heating rate of the non-isothermal scan. According to the equations, the Ea and A where β represents the heating rate of the non–isothermal scan. According to the equa2values can be determined from the slope and intercept of the linear fitting plots of ln( /Tp ) tions, the Eα and A values can be determined from the slope and intercept of the linear versus 1/Tp and ln(b) versus 1/Tp, as presented in Figure 2d,e. With the content of TGIC fitting plots of ln(β/Tp2) versus 1/Tp and ln(β) versus 1/Tp, as presented in Figure 2d,e. With in BCEP-TGIC increasing, Ea calculated using the Kissinger equation (Eak) and the Ozawa the content of TGIC in BCEP–TGIC increasing, Eα calculated using the Kissinger equation equation (Eao) of the system increases; e.g., the Eak and E o of BCEP1-TGIC3, BCEP1-TGIC1, a(Enαdk) and the

BCEP3-TOGzICaw1aareeq7u0a.6tioannd(E7α4o.)1okfJthmeoslys1t,e7m3.2inacnreda7s6e.s5; ek.Jgm., othle1E, αaknadnd57E.4αoaonfdB6C1E.4Pk1–J TGIC13,, BreCsEpPec1–tiTvGelIyC.1T, haenrdefBoCreE, Pth3–eTiGncIoCr1paorrea7ti0o.n6 aonfdTG74IC.1 ckaJnmloowl−1e,r7t3h.2e acnudrin7g6.5acktiJvity of mol−1, mol tahnede5p7o.4xyansydst6e1m.4. kTJhmisopl−h1e,nreosmpeecntoivneclya.nTbheeerxefpolraein,ethdeaisnfcoolrlpoworsa.tion UsuoafllTy,GcIyCclcoaanliplohwateicr ethpeoxciudreinsghaavcetivaihtyigohferthreeaecptiovxiytystyosatenmhy.dTrhidiseps htheanno mgleyncoidnyclaenthberes,exbpeclaaiunseedthaesyf otellnodwtso. oetfhTerGsI,Cbe,chaauvsientgheaystternodngtoerliencgtroopne-wniinthgdmraowreinegasialbyialittyh,igdhecteremapseesratthuererseathcatinvitthye olaftttehre. gInlyacdiddyitlioetnh,etrhweiisthocaynahnyudrraitdeeocfuTrGinIgCa,gheanvtisn[g26a,3st1r]o.ng electron–withdrawing ability, decreases the reactivity of the glycidyl ether with anhydride curing agents [26,31].

Curing System

samples show a relativ1e0ly high L1O6I7.v3alue, although the cured TGIC resins underperform inBflCaEmP1e–TrGetIaCr1dancy (h1a5ving an 1L7O5.I3 value of732.32.2%). Am1o2n.3g the m0.9,1the cured BCEP176.5

addition, it shows a relatively low FIGRA value of 7.5 kW m 2 s 1 (FIGRA of ERL-4221 itatively evaluate the flame retardancy of the cured resins according to the results of the is 12.3 kW m 2 s 1). This indicates that the flame-retardant epoxy system presents a CCT. Figure 3 depicts the curves of heat release rate (HRR), total heat release rate (THR), relatively low heat release rate and fire spread rate under simulated fire conditions [34,35]. and total smoke release rate (TSP). Some important parameters obtained from the test are

listed in Table 3, including the time to ignition (TTI), peak of HRmR2(PvHerRsuRs),3t0i.m5 e to PHRR -TGIC3 show a higher TSP than the cured ERL-4221 (39.1 m2) and it B(TCTEPPH1 RR), THR, TSP, fire growth rate (FIGRA = max value of HRR(t)/t), and char resifinally achieves more char residues than the contrast sample (0 versus 4.8%). According dues. BCEP1–TGIC3 present a little longer TTI and TTPHRR than ERL–4221, indicating to the above results, the cured BCEP1-TGIC3 outperforms the commercially available that they perform well in delaying ignition and fire growth during the initial stage of the

test, due to the introduction of flame–retardant groups. Usually, PHRR is considered as electrical and electronic fields. the most important parameter to evaluate the fire–safety performance of the material T[3ab2,l3e32]..FItorimsnuolateadndthflaatmBeCreEtPar1d–TanGcIyCte3satcrhesieuvltessoaf tlhoeweeproxPyHsRysRteamn.d THR than the reference sample; in addition, it shows a relatively low FIGRA value of 7.5 kW·m−2 s−1 (FIGRA of sen/ts a relative/ly low heat2r8e.l5e6ase rate and/ fire spread r/ate under sim1u8.l0ated fire cNo.Rnd.citions [343,375.8]0. In addi/tion, due to2t8h.6e0incomplete4.c8o2mbustion /of the flame–r2e2t.a4rdant mNat.eRr.ial, the cur1e1d.34BCEP1–T3G.78IC3 show1a5.h12igher TSP th3.a1n7 the cured0.E5R9L–4221 (3293..19 m2 versuNs.R3.0.5 m2) and3.7it8 finally a3c.7h8ieves mo9r.e41char residue1s.8t9han the co1n.0t5rast sampl2e4(.20 versus N4..8R%.). Accor3d.7in8g to the 1a1b.o34ve resul2ts2,.0t1he cured BC0E.8P61–TGIC3 o1u.t4p4erforms th2e5.c2ommerciNa.lRly. avail/ 29.70 50.40 / 1.75 23.2 N.R. able ERL–4221 in flame retardancy, and thus exhibits the potential for versatile applicaaticoanlcsuliantedelpehcotrspichaolruasncdonetleenct;tbrocanlicculfaiteelddnsi.trogen content; c no rating.

ERL-4221 (g) ERBLC–4E2P2(1g)is 12T.G3 IkCW(g·)m−2 Ms−1e)H.THhPiAs i(ngd)icaPteas( wtht.a%t )the fNlabm(ew–tr.%et)ardaLnOt Ie(p%o)xy syUsLte-9m4 pretbhye tchoenceocnaelocrailmoreitmere.ter.

T (°C) i 347 335 291 BCEP1–TGIC3 12.3 7.5 4.8 2.4. Thermal Behaviors of the Cured Epoxy System

TG was conducted to study the thermal stability of the cured epoxy system. Th6eoTf1G2 and derivative TG (DTG) curves under nitrogen and air atmospheres of the cured ERL– 4221 and BCEP–TGIC samples are plotted in Figure 4, and the data collected from the curve are listed in Table 4. Under the nitrogen and air atmospheres, the cured BCEP–TGIC and BCEP-TGIC samples are plotted in Figure 4, and the data collected from the curve are samples present a one–stage thermal weight loss course ranging from 250 °C to 400 °C, listed in Table 4. Under the nitrogen and air atmospheres, the cured BCEP-TGIC samples which is mainly attributed to the degradation of the cured epoxy networks. When the present a one-stage thermal weight loss course ranging from 250 C to 400 C, which is content of TGIC in the BCEP–TGIC increases, the initial weight–loss Wtemhepnerthateucroen(Ttei)natnodf mainly attributed to the degradation of the cured epoxy networks.

TGIC in thewBeCigEhPt-–TlGosIsC tienmcrpeearsaetsu,rthee(Tinmiatxi)alowfetihgeht-loss temperature (T ) and maximum maximum cured resin improve; e.g., the cured i wBCeiEgPh1t–-lToGssICte3mshpoewrastuareTi(Tof 31)8o°fCthaencduareTdmraexsoi nf 3im87p°rCovuen;ed.egr.,tthhee nciutrreodgeBnCaEtPmo-TspGhIeCre max 1 3 sahnodwpsraesTenotfs 3a18Ti Cofa3n3d5 a°CT andofa3T8m7axCofu3n8d2er°Ctheunditerorgtehne aatimroatsmphoesrpehaenrde,pinredsiecnattsinagTa i max i ogfo3o3d5thCeramndalastTabilitoyf o38f2theCfluanmdee–rrtehtearadiraanttmeopsopxhyesrye,sitnemdi.caNtiontge athgaotowdithetrhme aclosnttaebniltitoyf max oBfCtEhPe flinacmreea-srientagr,dthanetrespiodxuyalsywseteigmh.t Nofottheethcuatrewditrhestihnes caotn7t0e0nt°Cof(BWC70E0)Peixnhcirbeiatsinagtr,etnhde roefsiddeucarelawsienigh(tloowftehrethcuanred10r%es)i.nTshaits 7is00beCca(uWse th)eexphhiobsiptshatterebnodnodfs doefcBreCaEsiPngar(elovweeryr 700 that the crosslinking density greatly influences the thermomechanical behavior of a resin. behavior of a resin. BCEP–TGIC systems with a higher content of bi-functional BCEP pre

sent a lower Tg, while the cured BCEP1–TGIC3 shows a relatively high Tg of aboutg200 °C. the cured BCEP -TGIC shows a relatively high T of about 200 C. According to the results, According to th1e resu3lts, it can be found that BgCEP–TGIC systems show good heat reit can be found that BCEP-TGIC systems show good heat resistance, with a relatively high sistance, with a relatively high Ti and Tg, even better than those of DGEBA–type resins

[ 3i6,37], gand thus, they can tolerate a high manufacturing and working temperature of electronic and electrical products. a high manufacturing and working temperature of electronic and electrical products. pheres.

TG (N )

2 T max

(°C) 372 382 356

7/of 12 / 2.5. Dielectric Properties of the Cured Epoxy System

epoxy system with degradation capacity. We believe the degradation of the cured BCEP and BCEP1-TGIC3 can be due to the hydrolysis and alcoholysis of the phosphate bond in

white particles in 3.5 h, while the cured TGIC keeps stable for 5 h under the same conditions, indicating that the BCEP endows the epoxy system with degradation capacity. We 8 of 12 believe the degradation of the cured BCEP and BCEP1–TGIC3 can be due to the hydrolysis and alcoholysis of the phosphate bond in NaOH methanol/water solutions. Int. J. Mol. Sci. 2023, 24, 2301 radation in a methanol/water solution of NaOH under 50 °C. degradation in a methanol/water solution of NaOH under 50 C.

FFiigguurree 66.. PPhohtootgorgarpahpshsofotfhethceurceudreBdCBECPE,BP,CBECP1E–PT1G-TICG3I,Ca3n,daTnGdITCGbIeCfobreefaonrde aafntderacfhteermcichaelmdiecga-l 3. MMaatteeriiallss and Methods

Phenylphosphonic dichloride (AR, 98%), 3–cyclohexene–1–methanol (AR, 98%), triethylamine (AR, 99%), 3-chloroperbenzoic acid (m-CPBA) (AR, 85%), dichloromethane ethylamine (AR, 99%), 3–chloroperbenzoic acid (m–CPBA) (AR, 85%), dichloromethane ((CCHH22CCll22))(A(ARR, ,9999.5.5%%),)a,nahnyhdyrdoruosus magnesium ) (AR, 99%), and sodium magnesium suslufaltfeat(eM(gMSgOS4O)(4AR, 99%), and sodium bicbaicrabrobnoanteat(eN(NaHaHCOCO3)3and TGIC (AR, 98%), were provided by Macklin Chemical Reagent ) and TGIC (AR, 98%), were provided by Macklin Chemical Reagent

Co. Ltd., Shanghai, China. 3, 4–Epoxycyclohexene methyl 3, 4–epoxycyclohexenate (ERL– 4221) (AR, 97%) was obtained from Aladdin Biochemical Technology Co., Ltd., Shanghai, 4221) (AR, 97%) was obtained from Aladdin Biochemical Technology Co., Ltd., Shanghai,

China. MeHHPA (AR, 98%) was supplied by Chengdu West Asia Chemical Co., Ltd.,

Chengdu, China. All raw materials and chemical reagents were used as received without further purification. further purification.

Under a nitrogen atmosphere, 62.5g 3–cyclohexene–1–methanol (0.56 mol) and 38.2 triethylamine (0.26 mol) were added into a flask and dissolved in 100 mL CH2Cl . Then, 2 mL triethylamine (0.26 mol) were added into a flask and dissolved in 100 mL CH2Cl2. 36.26 g phenylphosphonic dichloride (0.19 mol) was slowly added to the solution in an Then, 36.26 g phenylphosphonic dichloride (0.19 mol) was slowly added to the solution ice bath. After stirring overnight at room temperature, the obtained mixture was washed in an ice bath. After stirring overnight at room temperature, the obtained mixture was with excessive water several times. Then, an organic solution was achieved using the washed with excessive water several times. Then, an organic solution was achieved using separating funnel and dried over anhydrous MgSO4. After filtration, the solvent and excess the separating funnel and dried over anhydrous M cyclohex-3-enyl-1-methanol were removed via distgilSlOat4i.oAnfutenrdfeilrtrraetdiounc,edthperseoslsvuernet, aanndd efixncaelslys, cbyisc(lcoyhcelxo–h3e–xe-3n-yeln–y1l–mmeeththyal)npohlewnyelrephreomspohvaeted (vBiCaPd)iwstailslaotbiotaninuenddaesr arebdruowcendlpiqrueisdswuirteh, aanydielfdinoafll9y6, %bi.s(cyclohex–3–enylmethyl)phenyl phosphate (BCP) was obtained as a browInnlaiqfluaidskwciotholaedyiienldanofic9e6b%a.th and equipped with a mechanical stirring, 34.6 g BCP (0.10Inmaolf)l,a4sk1.3cogolmed-CinPBanAic(0e.2b4atmhoaln)d,2e0q.2uigppNeadHwCitOh3a(0m.2e4chmanoilc),alasntdirr3i0n0g,m34L.6CgHBo2fCClP 9 122 (w0.e1r0emadodl)e,d41,.a3ngdmth–eCnPtBhAe m(0.i2x4tumreolw),a2s0.v2ioglNenatHlyCsOtir3r(e0d.24fomro12l),hanudnd3e0r0 amnLitCroHg2eCnl2awtmeroeAwfatsehrethdawt,itthheasNysateHmCOw3assoqluuetniocnheadndusdineigoaniszoeddiuwmatseur.lfTitheesno,luthtieoonr,gaanndicfuprhthaseer wwaasshceodldried with anhydrous MgSO4, and subsequently loaded onto silica gel column chromawleicttheda, NdraiHedCwOi3thsoalunthioyndraonuds MdegioSnOiz4,edanwdastuerb.seTqhueenn,ttlhyelooargdaendiconpthoaseiliwcaagsecloclloelcutmedn, tography. Finally, the solvent was removed using a rotary evaporator, and 22.5 g of bis((7– chromatography. Finally, the solvent was removed using a rotary evaporator, and 22.5 g of oxabicyclo[4.1.0]heptan–3–yl)methyl) phenylphosphonate (BCEP) (yield 65%) was obbis((7-oxabicyclo[4.1.0]heptan-3-yl)methyl) phenylphosphonate (BCEP) (yield 65%) was tained as a yellow liquid with a viscosity of 30 Pa·s at 25 °C. Scheme 1 introduces the full obtained as a yellow liquid with a viscosity of 30 Pa s at 25 C. Scheme 1 introduces the full

to the ISO 5660 standard, in which the specimen dimension was 100 100 1.6 mm3 and the heat reflux was 35 kW m 2. TG analysis was performed by using a TA5500 instrument (TA, New Castle, Delaware, USA) under an air or nitrogen flow of 25 mL min 1 with a heating rate of 10 C min 1. DSC analysis was carried out on a TA2500 apparatus (TA, New

veloped cycloaliphatic epoxy systems (BCEP-TGIC) by mixing phosphorus-containing

that the BCEP-TGIC systems showed a high curing activity and they could be efficiently cured, although the incorporation of TGIC lowered the curing activity of the resin. Among the systems, the cured BCEP1-TGIC3 showed relatively good flame retardancy, with an

compared to the commercially available ERL-4221. TG tests confirmed that the cured BCEP-TGIC systems presented good thermal stability, in which the Ti and Tmax of the cured resin improved, with the content of TGIC increasing. The cured BCEP1-TGIC3 had a high dielectric performance, with " and tan that were even lower than the cured ERL-4221 over a wide frequency range of 1 HZ to 1 MHz. The cured BCEP1-TGIC3 showed good chemical degradation behaviors in an alkali methanol/water solution, in which BCEP endowed the resin with a degradation capacity. Using the advantage of good flame retardancy, high thermal stability, superior electrical insulation, and ease of degradation in mild conditions,

Supervision, funding acquisition. Y.-J.X.: Conceptualization, supervision, writing—review & editing, funding acquisition. All authors have read and agreed to the published version of the manuscript.

and National Key Research and Development Program of China (2021YFB3700201).