Thermal degradation of epoxy resins

Polymer International Polym Int 53:1901–1929 (2004)DOI: 10.1002/pi.1473

ReviewThermal decomposition, combustion andflame-retardancy of epoxy resins—a reviewof the recent literatureSergei V Levchik1∗ and Edward D Weil21Akzo Nobel Chemicals, 1 Livingstone Avenue, Dobbs Ferry, NY 10522, USA2Polytechnic University, Six Metrotech Center, Brooklyn, NY 11201, USA

Abstract: An overview of the recent literature on combustion and flame-retardancy of epoxy resins ispresented. A brief overview of the structures of cured epoxy resins is also presented as a backgroundfor better understanding of the thermal decomposition and combustion phenomena. The literaturesources were mostly taken from the publications of 1995 and later; however, for basic descriptions of thestructural and thermal decomposition principles, older publications are also cited. New developments inflame-retardant additives, epoxy monomers and curing agents are described. It is shown that the mainattention in recent years has been focused on phosphorus-containing epoxy monomers and epoxy resins.Silicon-containing or nitrogen-containing products and inorganic additives remain of great interest assupplementary materials to phosphorus flame-retardants. 2004 Society of Chemical Industry

Keywords: epoxy resins; thermal decomposition; combustion; toxicity of combustion gases; flame retardancy

INTRODUCTIONEpoxy resins are characterized by the presence ofepoxide groups prior to cure, and they may also con-tain aliphatic, aromatic or heterocyclic structures inthe backbone. Epoxy resins are relatively expensive;however, the long service time and good physi-cal properties often help by providing a favorablecost–performance ratio when compared to other ther-mosets. The main fields where fire-retardancy of epoxyresins is required are electronics (printed wiring boardsand semiconductor encapsulation) and transportation(automotive, high speed trains, military and commer-cial aircraft) in composite structural and furnishingelements.1 Like other thermoset resins, epoxy resinscan be rendered fire-retardant either by incorporatingfire-retardant additives or by copolymerization withreactive fire retardants. Additive-type fire retardantsare mostly used in coating or encapsulation, whereasreactive flame retardants are preferable in printed cir-cuit boards and composites in order to avoid the riskof deterioration of physical properties.

STRUCTURES OF EPOXY RESINSIn order to understand the thermal decomposition andcombustion of epoxy resins, we will briefly discuss the

main structural elements of the cured epoxy network.More detailed discussion of epoxy chemistry can befound in the books edited by May2 and Ellis.3 Thestructure of the cured resin depends on the epoxymonomer and curing agent used. Usually, chemicallinkages generated by reactions of glycidyl ethersare less stable than other chemical linkages in theepoxy network, and therefore there is justificationto discuss here only those structures formed by theglycidyl ethers.

Carboxylic acids easily react with epoxies; however,generally anhydrides are used for curing (Scheme 1).

Anhydrides of dicarboxylic acids produce linearstructures with diepoxides, and crosslinking usuallyoccurs due to esterification of the alcohol groups(Scheme 2).

Aliphatic and aromatic diamines are the mostwidely used classes of curing agents. With theproper catalyst, aliphatic diamines cure epoxies atroom temperature, whereas elevated temperatures arerequired for aromatic diamines (Scheme 3).

Secondary amines, although more hindered thanprimary amines, can still react with epoxies and formcrosslinks.

Aliphatic alcohols are reactive with epoxies; how-ever, they are not normally used as curing agents,

∗ Correspondence to: Sergei V Levchik, Akzo Nobel Chemicals, 1 Livingstone Avenue, Dobbs Ferry, NY 10522, USAE-mail: [email protected](Received 24 February 2003; revised version received 9 May 2003; accepted 3 September 2003)Published online 13 October 2004

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Scheme 1.

Scheme 2.

Scheme 3.

whereas polyphenols, which are becoming more fre-quently used because they favor a high glass transitiontemperature of the cured epoxy (Scheme 4).

In the presence of catalyst, epoxies can undergoself-curing. This process always occurs to some extentwhen the curing agent is used, and mostly concernsthe reaction of a secondary alcohol with the epoxy(Scheme 5).

Self-curing of epoxies is often responsible forcrosslinking when difunctional curing agents areused.

THERMAL DECOMPOSITION

The base resinThe thermal stability of epoxy resins, as well astheir flammability, depends on the structure of themonomer, the structure of the curing agent and thecrosslink density. The literature data on the effect ofcrosslink density on thermal stability are contradictory.For example, Dyakonov et al4 showed that the thermalstability increases with increasing crosslink density

for the analogous resins. On the other hand, Iji andKiuchi5 found that the ‘pyrolysis resistance’ of novolacepoxy resins was slightly decreased by the additionof an excess of hardener. In general, the thermalstabilities of aromatic epoxy resins are higher thanthose of aliphatic ones, even though the crosslinkdensities of the aromatic networks may be lower. Thecopolymerization of aliphatic or aromatic epoxy-aminecured resin with self-cured resol novolac resin leadsto an increase of crosslinking and thermal stability;however, the level of the epoxy-amine system shouldbe kept below 15 wt%.6

Christiansen et al7 observed that epoxies cured withdicyandiamide (DICY) performed relatively poorlyin thermal stability tests, while epoxies cured withnovolacs had the best thermal stability. This wasexplained by the fact that the amine linkages presentin the cured epoxy exhibit a lower thermal stabilitythan ether linkages.

Usually, thermal decomposition of any epoxyresin starts from the dehydration of the secondaryalcohol, leading to the formation of vinylene ethers8–21

(Scheme 6).

Scheme 4.

Scheme 5.

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Decomposition, combustion and fire retardancy of epoxy resins

Scheme 6.

Scheme 7.

Scheme 8.

Scheme 9.

The resulting allylic ether C–O bond is thermallyless stable than the original C–O, and thereforechain scission mostly occurs at the allylic position.In epoxies copolymerized with amines, the allylicamine C–N bond is less stable than the allylic etherC–O bond, and therefore in general, the amine-cured epoxy resins are less stable than anhydride-curedepoxies.22 It was found that epoxies cured with meta-substituted aromatic amines are more thermally stablethan epoxies cured with para-substituted aromaticamines.4 Both homolytic and heterolytic chain scissionof allylic C–N (C–O) bonds were suggested in theliterature,13,16,18,23 which leads essentially to the sameresult (Scheme 7).

A second similar chain scission with the secondaryamine group (C–NH) might be expected to result inliberation of the curing agent. However, this seems notto be the case, since amines were found only as a minorproduct of the thermal decomposition of epoxies.24

Amines usually volatilize as a part of the chainfragments or stay in the solid residue and undergocharring. On the other hand, phthalic anhydride wasregenerated in large quantities on thermal decomposi-tion of anhydride-cured epoxy resins22,25 (Scheme 8).

Upon further decomposition, aliphatic chain endsproduce light combustible gases, allyl alcohol, acetoneand various hydrocarbons.14–16,18,20–22

Alternatively, the allylic ethers or amides, formedafter losing water, can undergo the Claisen rearrange-ment (Scheme 9) which changes the paraphenylenegroup to a 1,2,4-trisubstituted benzene with increasedthermal stability.26 This structure is partially respon-sible for further crosslinking and charring of epoxyresins.

In addition to the main processes of chain scissiondiscussed above, many secondary processes, whichlead to minor products of thermal decomposition,have been reported in the literature.12–16,18 Forexample, cyclization of aliphatic chain ends, insteadof splitting off, can contribute to the charringand fire retardancy8–10,13–15,18,20 (Schemes 10 and11).

At high temperatures, polyaromatic hydrocarbons,such as naphthalene and phenanthrene, are impor-tant degradation products formed by ‘pyro-synthesis’and aromatization reactions.25 Radical recombinationis also a possible pathway for the formation of highlyaromatic compounds. Biphenyl could be formed bycombination of two phenyl radicals. Hydroxydiphenyl-methane could originate from the combination ofa phenyl radical with a hydroxyphenylmethyl radi-cal or a phenylmethyl radical with a hydroxyphenylradical.

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Scheme 10.

Scheme 11.

Thermal oxidationThermal oxidative decomposition of the epoxyresins has also been extensively reported in theliterature.11,27–31 Basically, three mechanisms for theoxidation of epoxies were suggested: (1) attack ofoxygen on the methylene group,11 (2) oxidation ofthe tertiary carbons in the aliphatic portion of thechain, which is usually an ester-type crosslink in theanhydride cured resins,29 and (3) oxygen attack onthe nitrogen in the amine-cured epoxies.27,31 Any ofthese mechanisms leads to the formation of carbonylgroups (isomerization in the case of (3)) which furtherdecompose and result in chain splitting.

Kinetics of thermal decompositionThe kinetics of the thermal decomposition of epoxyresins in relation to their flammability and to modelcombustion has been reported in the literature.32–35

These studies applied the method of the invariantkinetic parameter (IKP)36, in order to describe thedegradation of the resins. This method allows theevaluation of the probabilities for various kineticfunctions and thus to describe the degradationmodel of the material. The kinetic model whichbest describes the decomposition of epoxy resinsis a diffusion-mechanism model. Comparison of theinvariant activation energies of epoxies cured withdiaminodiphenyl sulfone (DDS) and dicyandiamide(DICY) showed that the resin cured with the aromaticdiamine is more stable towards thermal oxidation.Similarly, Le Huy et al30 showed that thermaloxidative decomposition of epoxies at relatively lowtemperatures is diffusion-controlled because of theneed for oxygen penetration to the deep layers of theresin.

Decomposition in the presence offlame-retardantsPaterson-Jones et al15 showed that commonly usedinorganic fillers, eg alumina or silica, accelerate thethermal decomposition of epoxies, probably becauseof catalysis. Kinetic evidence of the catalytic actionof Al, Cu or Zn used as fillers in epoxy resins wasprovided by Sanchez et al.37

Scheme 12.

The thermal degradation of a brominated epoxyresin consisting of the diglycidylether of bisphe-nol A (DGEBA), chain-extended with tetrabro-mobisphenol A (TBBA) and cured with 4,4′-diaminodiphenylsulphone (DDS), has been investi-gated by Luda et al.38 Thermolysis occurred in threesteps: decomposition of the brominated part of theresin, decomposition of the non-brominated part andchar formation. Brominated aliphatics, mono- anddibrominated phenols were released in the first stage,whereas HBr was found in the gaseous products onlyabove 330 ◦C. The non-brominated part of the resindecomposed with evolution of unsubstituted and alkyl-substituted phenols, bisphenol A and alkoxyaromatics.Nitrogen-containing groups were found to accumulatein the residue due to the high level of crosslinking.

The kinetics and mechanisms of the thermal degra-dation of a phosphorus-containing epoxy, based onbis-(3-glycidyloxy)phenylphosphine oxide (BGPPO)(Scheme 12) and 4,4′-diaminodiphenylsulphone(DDS), were studied by Liu et al.39 The degradationof the epoxy resulted in high char yields and residueswith high phosphorus contents. The phosphorus con-tent of the residues of thermal degradation was foundto increase from 6.2 wt% at 200 ◦C to 9.7 wt% withincreasing temperature to 550 ◦C. It was suggestedthat phosphorus plays an important role in fixing thecarbon, to reduce the production of flammable gasesand to form a phosphorus-rich residue.

The thermal properties of epoxy resins contain-ing flame-retardants based on silicon, phosphorusand melamine, were investigated by using thermo-gravimetric analysis.40 Phosphorus groups loweredthe initial decomposition temperature of the epoxyresin, whereas silicon and melamine groups did not.The integral decomposition temperatures of the epoxyresins were significantly increased with simultaneousincorporation of phosphorus and silicon, owing tothe formation of highly thermally and oxidatively sta-ble char which led to good char preservation at hightemperatures under air. The low char yields of theepoxy resins containing only silicon also implied thatsilicon could serve as a ‘char-protector’, but not a pro-moter for ‘char-formation’. Incorporating melaminegroups into the silicon-containing epoxy resins didnot alter the resin’s thermal stability and degradationcharacteristics.

COMBUSTIONEpoxy resins are very reactive, which allows versatilityof curing agents, either catalytic or reactive. Thecatalytic curing agents do not build themselves into

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the thermoset structure and therefore do not muchaffect the flammability of the resin. For example, itwas shown41 that epoxy resins catalyzed by variousboroxines have essentially the same Oxygen Index (OI)because the crosslinking densities were comparable.On the other hand, some enhancement of the oxygenindex was observed upon increasing the amount of thecatalyst and a very significant increase was observedupon increasing of curing time, which is attributed tothe increase of the crosslink density.42,43 In contrast,Macaione et al44 found no essential change in theOI once the system has been through the initialcure portion of the cure cycle so that the presenceor absence of post-curing makes no contribution tothe flammability characteristics of the material. Avery high crosslink density of the epoxy resin couldmake the network structures too rigid to produceintumescent charred layers during combustion andtherefore deteriorate flame retardancy.45 The charsof highly crosslinked resins instead cracked andcontinued burning.

Reactive curing agents mostly represented byamines, anhydrides or phenolic resins strongly modifythe flammability. The combustion behavior of simi-lar epoxy resins depends on the ratio of oxygen tocarbon atoms in the polymer structure. Epoxy resinscured with amines tend to produce more char and theyapparently are less flammable than acid- or anhydride-cured resins at comparable crosslink densities.46 How-ever, nitrogen can be supplied also from the epoxymonomer. For example, a self-cured tetraglycidyldiaminodiphenylmethane-type resin containing nitro-gen in its structure is less combustible20 than the sameresin cured by diaminodiphenyl sulfone.21

Lin and Pearce47 and Chen et al48 showed thatthe OI of epoxy resins correlates with the charringperformance. Le Bras et al35 found correlationsbetween OI and peak of heat release rate and ignitiontimes measured in cone calorimetry. Typically, non-flame-retardant resin ignites at about 45–60 s andextinguishes at about 200–220 s at a heat flux of35 kW m−2. After 200 s, epoxy resins usually show‘post-glowing’ combustion. The combustion of theepoxies in the cone calorimeter is not complete andleads to the formation of thermally stable residues.Typically, epoxy resins are more flammable thanphenolic resins used in the same application49 andtherefore epoxy resins require flame-retardants.

FLAME-RETARDANT EPOXIES

Inherently flame-retardant epoxiesEpoxy resins cured by phenol formaldehyde resinare to some degree inherently flame-retardantbecause of the significant charring tendency ofthe phenolic component. The char yield can beincreased by using special epoxy monomers contain-ing highly aromatic bisphenols (eg phenolphthalein,bisphenol-fluorenone,41,47,50 bisphenol-anthrone and

Scheme 13.

Scheme 14.

Scheme 15.

Scheme 16.

tetraphenol-anthracene48) or those containing dou-ble bonds able to undergo the Diels–Alder reaction(eg mono-, di- or trihydroxystyrylpyridine).42 Thesemonomers were cured either alone or in combinationwith regular grade epoxy monomers (eg the com-mercial diglycidyl ether of bisphenol A (DGEBA)),resulting in a higher crosslink density and showing anincrease of OI from 20 for DGEBA to almost 40 forhighly aromatic monomers.

Compounds that contain novolac derivatives (phe-nol novolac-type (Scheme 13) or o-cresol novolac-type(Scheme 14) epoxy resins) including aromatic groupsin the main chain display far higher flame-retardancythan that of epoxy resin compounds without aro-matic groups.5,45,51 Of the two aromatic groupsincluded in the chain, the biphenylene group (a mix-ture of 4,4′-diglycidyl-(3,5,3′,5′ tetramethyl-biphenyl)ether (Scheme 15) and 4,4′-diglycidyl-biphenyl ether(Scheme 16) (50 + 50 wt %)) was more effective thanthe phenylene group. However, the inclusion of anon-aromatic group, a phenol dicyclopentadiene-typeepoxy resin (Scheme 17), in the chain had no flame-retardant effect.

Epoxy resin compounds containing the biphenylenegroup in the novolac structure (Scheme 18) were moreeffective than compounds containing the phenylenegroup (Scheme 19).

It was speculated that the biphenylene-containingresin resulted in lower crosslinking densities and higher

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Scheme 17.

Scheme 18.

Scheme 19.

Scheme 20.

elasticity which facilitated formation of an intumes-cent layer. Epoxy resins containing these novolacderivatives with aromatic bridging groups showedrelatively high pyrolysis resistance while having rel-atively low crosslink densities. The high pyrolysisresistance appeared to have made an important con-tribution to the stability of the intumescent foamlayers during combustion. As previously noted, theinclusion of a non-aromatic group (dicyclopenta-diene) in the novolac resins failed to form anyprotective foam layer in spite of the low crosslinkdensity of the epoxy resin compound. This wasbelieved to be due to the lower pyrolysis resis-tance of the compound with the non-aromaticgroup.

In addition to the highly charrable biphenyleneepoxies shown above, an epoxy having a double bondin its structure (Scheme 20), a naphthalene-basedepoxy (Scheme 21)52 and epoxy resins containing tert-butyl-substituted aromatics (Schemes 22 and 23)53

were investigated.Typically, these resins cured with biphenol-

(Scheme 24) or naphthalene- (Scheme 25) basednovolacs showed a V-1 UL 94 rating without any

Scheme 21.

Scheme 22.

Scheme 23.

Scheme 24.

Scheme 25.

Scheme 26.

auxiliary flame-retardant additive, or V-0 in the sys-tems heavily loaded with silica.

The use of a multifunctional epoxy resin with fourglycidyloxy groups, tetrakis (glycidyloxyphenyl)ethane(Scheme 26), in combination with other epoxy resincompounds improved the thermal stability whilestill maintaining excellent flame-retardancy.5,54 It isbelieved that this combination of results could beattributed to a local increase in crosslink density.Although the overall elasticity slightly decreased, itremained high enough for intumescent layer forma-tion. Another highly crosslinkable epoxy performedsimilarly (Scheme 27).54

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Scheme 27.

A high-performance epoxy-imide polymer wasobtained by curing diimide-diepoxide (Scheme 28)with 4,4′-diaminodiphenyl ether (DDE), 4,4′-di-aminodiphenylsulphone (DDS), tris(3-aminophenyl)phosphine oxide55 or bis(3-aminophenyl)methyl-phosphine oxide.56 The thermal and flame resistancesof epoxy resins were significantly improved by theintroduction of imide groups into the epoxide struc-ture.

In order to prepare high-performance epoxy resins,Hwang and Jung57 used three types of diamines: N,N ′-(4,4′-diphenylether)-bis(4-aminophthalimide) (Sch-eme 29), 4,4′-bis(p-aminophenoxy)dibenzalpent-aerythritol (Scheme 30) and 2,2′-bis [4-(p-amino-benzoyl)phenyl]propane (Scheme 31) to cure DGEBAepoxy resins. The char yields measured thermogravi-metric analysis (TGA) at 500 ◦C were 49.4, 46.2 and34.5 wt% for these diamines, respectively, whereasthe char yield of the epoxy cured with methylenebis-dianiline was 32.9 wt%. The high char yield ofthe imide-containing amine implies that incorporatingimide into the epoxy matrix moiety improves the flameretardancy of the epoxy.

In order to provide better flame-retardant perfor-mance and physical properties, epoxy resins can becopolymerized with other thermoset resins. For exam-ple, copolymerization of an aliphatic epoxy resin,partially cured by an aliphatic polyamine, with resolnovolac phenolic resin, provided a significantly betterflame-retardant material than the plain epoxy.58 In thecase of a composite material made with the pure epoxy,the char was formed but the fibers were exposed at thesurface, due to delamination which was attributed to

Scheme 29.

Scheme 31.

stresses produced by trapped gases within the compos-ite. The delamination of the composite during burningwas avoided in the presence of resol novolac. Tyberget al59,60 prepared void-free networks from the reac-tion of phenolic novolacs with various epoxides. Thesesystems had significantly improved mechanical prop-erties over commercial novolac networks and werefound to be comparable in fire resistance to typicalphenolics as measured by cone calorimetry.

A new ternary system based on epoxy, benzoxazineand phenolic resin was studied by Rimdusit andIshida.61 It was found that the combined systemshows a relatively high decomposition temperature,ie up to 370 ◦C, compared with about 270 ◦C forthe polybenzoxazine used. The material had improvedthermal stability with an increasing mass fraction ofepoxy in the system, which may be attributed to agreater crosslink density. On the other hand, the charyield of the ternary systems was significantly higherwhen compared with the pure epoxy resin. This is dueto the fact that both polybenzoxazine and phenolicnovolac are known to give a higher char yield whencompared with the epoxy resin.

A new advanced thermoset material was developedby copolymerization of special low-molecular-weightpoly(phenylene oxide) (PPO) and epoxy resins.62 Thelow-molecular-weight PPO resin provides excellentprocessability and compatibility with epoxy resins, incontrast to the high-molecular-weight resin which istraditionally used as a component in blends with high-impact polystyrene. In addition to contributing goodheat resistance and electrical properties to the epoxyresins, the low-molecular-weight PPO also improvesdimensional and hydrolytic stability, chemical resis-tance and provides flame retardancy. The electronic

Scheme 28.

Scheme 30.

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Scheme 32.

industry is also practicing copolymerization of epoxyresins with isocyanurate esters in order to achieve highelectrical performance and good flame-retardancy.63

Halogenated flame retardantsTetrabromobisphenol A (TBBA) (Scheme 32) is aunique reactive flame-retardant additive widely usedin epoxy resins, especially in electronic grades, whereflame-retardancy is mandatory. Normally, TBBA ispre-reacted with epoxy in the so-called chain extensionprocess. In order to obtain a V-0 rating in printedwiring boards, 20 to 37 wt% of TBBA is used. TBBAis reportedly the highest-volume brominated productsold in the market.

Although TBBA does not create obvious toxicolog-ical problems, recently it fell under attack by beingclassed together with halogenated additive products.64

The move to replace halogenated materials is gath-ering momentum with some Japanese and European,particularly Scandinavian, original equipment manu-facturers (OEMs).65 The thermal stability of bromi-nated epoxy resins has improved since the 1970sby proper formulation with special grades of ther-mally stable epoxy resins; however, there is still asignificant desire to obtain better thermal expansioncharacteristics and delamination performance, espe-cially in the light of the move to lead-free solderingwhich requires higher temperatures.66 Combinationsof low-brominated novolac and epoxy TBBA have theadvantage of low moisture uptake.67

Highly thermally stable 2,2′,6,6′-tetrabromo-3,3′,5,5′-tetramethyl-4,4′-biphenol (Scheme 33) havingthe m-brominated phenol moiety was synthesizedand reacted into epoxy resin systems.68 In electronicencapsulation and laminate applications, epoxy sys-tems derived from this brominated biphenol haveexhibited superior hydrolytic and thermal stability ascompared with the conventional o-brominated epoxyresins. These properties have resulted in an extendeddevice life for semiconductors and a high glass transi-tion temperature (Tg) with excellent blister resistancefor printed circuit boards, while meeting flame retar-dancy requirements as well.

Low smoke formation and low rate of flame spreadare important factors in structural composite materials.Bis(hexachlorocyclopentadieno)cyclooctane (Dechlo-rane Plus) (Scheme 34) in combination with antimonytrioxide provides an efficient flame-retardant levelin composite materials.69 Alternative synergists, zincborate or iron oxide, which partially or completely sub-stitute for Sb2O3, allow significantly reduced smokerelease and make Dechlorane Plus an attractivereplacement for brominated flame retardants, wher-ever smoke is deemed important.

Scheme 33.

Scheme 34.

The phthalide-containing epoxy resins were syn-thesized by chain extention with 4,5,6,7-tetrabromo-phthalein (Scheme 35) and characterized by compar-ison with TBBA epoxy resins in terms of thermalproperties.70 Although both resins contain comparableamounts of halogens, the resulting flame-retardancywas higher in the phthalide-containing resin. The charformation upon pyrolysis was also enhanced by thephthalide functionality.

The flammability, thermomechanical properties andfire response of the diglycidylether of 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene (DGEBC) (Scheme36) cured with several hardeners were recently exam-ined and compared to diglicidylether of bisphenol-A (DGEBA) systems.71 The mechanical propertiesof the DGEBC and DGEBA systems were equiv-alent, but the DGEBC systems exhibited superiorflame-resistance and 50 % lower heat-release rate andheat-release capacity than the corresponding DGEBAsystems. DGEBC cured with methylenedianiline hadan oxygen index of 30, exhibited UL 94 V-0/5V behav-ior and easily passed the FAA heat release requirementFAR 25.853(a-1) as a single-ply glass fabric laminate.The excellent fire-retardant performance of DGEBCwas attributed to its unique charring mechanism ratherthan to the effect of chlorine present in the molecule.

Phosphorus-containing additivesCured epoxy resins have a high concentration of OHgroups,1,20,21,72 and therefore phosphorus-containingfire-retardants are particularly effective in epoxy resinsbecause phosphorus-containing products tend to reactwith OH groups. Spennger and Utz73,74 showed thatthe amount of phosphorus needed to achieve a V-0 rating depends strongly on the type of hardenerused, as well as on the presence or absence of fibersor fillers. Anhydride hardeners require up to 5 % P,and dicyandiamide (DICY) usually 3 %. However,for laminates with 60 % fiber content 2 % P canalready be sufficient. As the amount of phosphorusneeded to fulfill the flammability requirements isdifferent for each epoxy application, every systemshould be optimized by an iterative test series. Epoxyresins can be fire-retarded by using conventionaladditives; however, reactive comonomers often aremore preferred because they allow the maintaining

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Scheme 35.

Scheme 36.

of better physical properties. An extensive reviewon phosphorus-containing flame-retardants in epoxyresins was recently published by Jain et al.75

The advantage of phosphorus-based printed wiringboards (PWBs) is that their tracking/arcing perfor-mance is significantly improved, although other keyproperties may suffer.65 Water absorption could berelatively high and likely to cause problems in the man-ufacture of ‘plated-through’ holes in the printed circuitboards. Moisture ingress into the laminate may beencapsulated by ‘plating’ down the hole, and can sub-sequently cause blistering of the plating when heated,for example, during soldering.

Red phosphorus is a very efficient additive for epoxyresins; however, it needs to be stabilized in order tomaintain long-term reliability. For example, 4 wt%of red phosphorus, coated with aluminum trihydrate(ATH) and encapsulated with phenolic resin, wasused in combination with 25 wt% ATH in an adhesiveformulation made from a blend of bisphenol A andcresol novolac epoxies.76 A V-0 rating in the UL 94test was observed with printed circuit board laminatesmade with this adhesive. Only 0.5 wt% of encapsulatedred phosphorus was required in a cresol novolac resinfilled with 500 parts of silica in order to obtain a V-0rated resin for packaging of electronic devices.77

Ammonium polyphosphate (APP) can be used insome epoxy formulations where long-term hydrolyticinstability can be tolerated.78 Low smoke generationof an epoxy containing APP is an advantage.Combinations with ATH help to improve somephysical properties while maintaining the requiredlevel of flame-retardancy. Although APP shows veryhigh efficiency in epoxy resins (15 wt% increases theOI from 22 to 31 in bisphenol A epoxy cured with analiphatic amine), co-addition of the potassium salt ofdiphenylsulfonedisulfonic acid helps to boost the OI to38.79 Apparently, APP in combination with ATH wasused in early commercial halogen-free printed wiringboards.80

Recently, Clariant81 patented technology on theuse of the aluminum salt of diethylphosphinic acid(Scheme 37) in combination with ATH or APPor melamine to flame-retard epoxy adhesives. Forexample, use of 10 parts per hundred parts ofrubber (phr) of the aluminum salt and 50 phr of

Scheme 37.

Scheme 38.

ATH resulted in a V-0 rating of the glass-reinforcedlaminate. In a patent to Chisso,82 it was suggestedto partially decompose APP to release 5–10 % ofthe stoichiometric ammonia, and then react the thus-formed acid groups with melamine. APP coated withmelamine provided a V-0 in bisphenol A type epoxyat 20 wt% loading.

Kodolov et al83 reported a study on intumescentcomposites based on epoxy resins crosslinked withpolyethylene–polyamine and containing APP andsuch co-additives as calcium borate, manganesedioxide and nickel- or chromium-containing tubulenes(products of dehydration and polycondensation ofphenanthrene containing Ni or Cr) as carbonizationstimulators. In fact, the introduction of metal-containing tubulenes led to compositions with lowflammability and high char yield, whereas the use ofcalcium borate considerably increased the strength ofthe intumescent foam being formed.

The ammonium salt of methylphosphonamidic acid(Scheme 38), encapsulated in polymethyldiethoxy-siloxane or polyaminopropylethoxysiloxane, was usedas a fire-retardant additive in composite materialscontaining 60 wt% of glass fiber reinforcements.84,85

At 15 wt% loading, the composite extinguished in airwithin 10 s.

The effect of triphenyl phosphate (TPP) and ATHon the flame-retardancy and thermal stability ofacid anhydride-cured epoxy resin was studied bythermogravimetry and OI.86 Unexpectedly, it wasfound that ATH, TPP and the mixture of ATHand TPP all could decrease the char residues of thecured resin, which is disadvantageous for the flame-retardant effect. Furthermore, it was shown that thisdisadvantageous effect is the most pronounced in thetemperature region of the resin’s skeletal degradation(about 360–450 ◦C). It was believed that the reactivealumina (a Lewis acid) which is formed as the ATHloses its water of crystallization, and the phosphorusacids formed as the TPP decomposes in the condensedphase, could catalyze the degradation of the curedepoxy resin.

A dual thermosetting system consisting of anepoxy resin, blended with an unsaturated polyesterwas flame-retarded either with triphenylphosphineoxide or various commercial aromatic phosphates(triphenyl phosphate, isopropylphenyl diphenyl phos-phate, cresyl diphenyl phosphate, etc.) or diethyl

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ethylphosphonate.87 It was found that in the pres-ence of a small amount of unreactive phosphorus-containing additives, the OI increased considerablybut the flame-retardancy was not further increasedby increasing the level of the phosphorus-containingadditive. In fact, a loading of 5 % of the additivesgave almost the same OI as 15 %. The presence ofphosphorus in the formulation contributed to theformation of an intumescent char. Only 7.5 wt%TPP was necessary to provide a V-0 rating in anovolac epoxy/bismaleimide blend cured by 4,4′-diaminodiphenylmethane.88

A comparison of DGEBA/DDS resins containingphosphate compounds as additives, namely, trimethylphosphate (TMP), triethyl phosphate (TEP), tri-butyl phosphate (TBP) and triphenyl phosphate(TPP), with resins including chemically bondedorganophosphate groups, namely, DGEBA pre-reacted with diphenyl or dialkylester phosphates, wasperformed by Derout et al.89 They showed that thefire-retardant behavior of crosslinked DGEBA/DDSwith incorporated dialkyl phosphate groups onto theepoxy resin backbone was always greater than thoseof the resins containing the corresponding trialkylphosphate additives. Moreover, phenyl phosphatederivatives lead to a good increase of fire retardancy(OI > 28): the comparison with the OI valueof a DGEBA/DDS specimen containing triphenylphosphate as additive proves that the best fire behaviorwas achieved when phenyl phosphate molecules arechemically bonded onto the resin backbone.

Triphenyl phosphine oxide at 10 wt% was foundto provide a V-0 UL 94 rating in the printed circuitboard laminates manufactured with high-performanceepoxy resins such as the blends of cresol novolac,phenolic alkyl novolac, triazine-modified novolac andxylene-modified novolac epoxy resins.90

Liu et al91 showed that the fire-retarding proper-ties (OI and UL-94) of a cured epoxy were stronglyimproved by using the commercial additive resor-cinol bis(diphenyl phosphate) (RDP). Moreover, acombination of the phosphate with phenolphthalein,used as a chain extender, gave substantially enhancedflame-retardant results. A related system where thephenolphthalein structure was built into the phosphateadditive (Scheme 39), rather than into the polymerbackbone, gave inferior results. Thermogravimetricanalysis (TGA) and infrared spectroscopy of the solidresidue suggested that the phenolphthalein group isinvolved in a crosslinking reaction, which may leadto char enhancement. The RDP depressed somewhatboth the glass transition temperature (Tg) and ther-mal stability, but the presence of the phenolphthaleinstructure partially overcame the Tg depression.

A series of hindered bisdiphosphates (Schemes 40–42) in combination with 80 % of amorphous silica wastested in an epoxy formulation for the encapsulationof electronic devices.92 A V-0 rating in the UL-94test was observed at 1.5 % phosphorus content in theepoxy adhesive.

Scheme 39.

Scheme 40.

Scheme 41.

Scheme 42.

Scheme 43.

Scheme 44.

Unexpectedly, antimony trioxide, a primary syn-ergist for halogen-containing flame-retardants butrarely effective with phosphorus, was found to besynergistic with bisphenol A tetraxylenyl phosphate(Scheme 43).93 For example, 16 wt% of the phos-phate and 11 wt% of Sb2O3 provided a V-0 rating inthe blend of novolac epoxy and bis(cyclohexyl)propaneepoxy (Scheme 44) filled with 10 wt% silica.

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Scheme 45.

Scheme 46.

Scheme 47.

Epoxy resins containing dimethyl methylphospho-nate (DMMP), which aids in the processing of theepoxy by reducing the viscosity of the resin mixture,were cured with amines.94 DMMP also acts as an‘antiplasticizer’ in the cured epoxy by increasing themodulus and yield strength. The flammability wasinvestigated by using a microcalorimeter, which mea-sures the amount of oxygen consumed during pyrolysisand provides a quantitative evaluation of the heatreleased upon combustion. The total heat releasedwas unchanged by the addition of the additive, henceindicating that the mechanism for degradation wasnot changed with the addition of DMMP. The lackof additional char formation is also an indication ofthis. However, the heat-release capacity is reduced ina manner similar to the decrease of degradation rateseen by TGA, from a peak of 1063 to 365 J g−1 K−1 at15 phr DMMP.

A phosphonate salt, made by the prolonged heatingof DMMP with urea, was added at an 11 wt% levelto DGEBA, and the resin, then cured with phenol-formaldehyde novolac, showed a V-0 rating.95 Thealuminum salt (Scheme 45), made by reacting methylmethylphosphonic resin with ATH, provided a V-0rating at 24 wt% loading.96 Salts made by heatingDMMP with salts of Mg, Ca, Zn, Ba or Sb wereless efficient and showed only a V-2 rating at 25 wt%loading.97

Epoxy-based laminates containing 14 wt% of thecommercially available cyclic phosphonate (Scheme46) showed a V-0 rating in the UL 94 test.98 Similarcyclic phosphonates (Scheme 47), where R is butyl,hexyl, octyl or 1,2-ethandiy/hexyl (oligomeric) groups,showed a V-0 rating at 25 wt% loading.99

The influence of different amounts of poly(propoxy-phosphazene) (Scheme 48) on the curing kinetics,physical properties and flame-retardancy of bisphenolA type-epoxies cured with diethylenetriamine wasinvestigated by Denq et al.100 The results revealed thatthe polyphosphazene was partially miscible with theepoxy and could be a catalyst or a diluent, depending

Scheme 48.

on the content. The tensile strength and the modulusof the blends decreased with increasing amounts of thepolyphosphazene, although the elongation increasedwith increasing flame-retardant (FR).

Mirkamilov and Mukhamedgaliev101 phosphory-lated a gossipol resin, which was a waste productof the fat and butter industry containing significantamounts of phenolic functionalities in its structure.Phosphorylation was carried out by using either phenylphosphates or phosphoramidates. It was found that thetime of ignition of the specimens and the OI valuessignificantly increased upon addition of the phospho-rylated resin to the epoxy. The increase in the contentof H2O and CO in the gas phase of the combustionproducts was accompanied by a significant reduc-tion in the concentration of hydrocarbons. This wasattributed to the fact that the burning gases undergooxidative decomposition inside the pre-heating zoneof the diffusion flame before they can reach the flamefront. However, the modified specimens also showedan increase in soot formation.

The use of tris(dichloropropyl) phosphate (TCPP)as a flame retardant in epoxy resins cured bymethyltetrahydrophthalic anhydride was studied byLi et al.102 The effect of the phosphorus contentof the TCPP on the OI of the epoxy resins wascompared with the results obtained when usingtriphenyl phosphate and trimethylphenyl phosphate.Although TCPP is an organophosphorus flame-retardant containing chlorine, it was not more effectivewhen compared to other flame retardants containingonly phosphorus. It appeared quite possible thatTCPP mainly works through phosphorus alone,despite the presence of halogen.

Maltseva et al103 measured the temperature on theback side of the sample heated at 10 W m−2 with alaser. The plots showed complicated shapes: a sharpincrease of the temperature at the initial momentcorresponded to heating of the composition before achar layer was formed, then an inflection of the curverelated to the formation and growth of the char wasobserved, and finally, a slow temperature increase,up to a stationary state, was indicative that the charformation process was finished. It was found that non-flame-retardant epoxy resin produces char slowly, andthat the protective properties of the char are low,whereas epoxy resins with phosphorus-containing FRstypically showed higher rates of char formation andbetter thermal insulative properties.

Phosphorus-containing epoxy monomersThere is a relatively large number of publications aboutreactive phosphorus-containing epoxy monomers.

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Scheme 49.

Scheme 50.

Series of diglycidyl phosphates, diglycidyl phospho-nates and glycidyl phosphinates have been preparedin the laboratories of Siemens AG.104 It was foundthat such phosphonates and phosphinates are moreefficient at the same level of phosphorus content thanthose phosphates, either incorporated into the resinnetwork, or added (tricresyl phosphate).

There are a large number of publications discussingthe use of 9,10-dihydro-9-oxa-10-phosphaphenan-threne 10-oxide (DOPO) (Scheme 49). This can beeither pre-reacted with epoxy resin or used as a reactiveadditive during curing. The glass-fiber-reinforcedlaminates prepared with DOPO could be classifiedV-1 with 1.6 % phosphorus and V-0 with 2.1 %phosphorus.73,74,105,106 With proper formulation, thestress at failure of these phosphorus-containing flame-retarded laminates, as well as their glass transitiontemperatures, can be similar to non-flame-retardantepoxies. The modulus and the strain at failure arecomparable with standard laminates. Investigations ofthe chemical resistance of DOPO-containing epoxysystems showed no large difference when comparedto the control, except for resistance to ammonia. Theweight gain when immersed in aqueous ammonia wassignificant for phosphorus contents above 2–3 %.

Phosphorus-containing epoxy resins (Scheme 50)(1–3 % phosphorus content) were synthesized byreacting DOPO with DGEBA.107,108 Higher OI valueswere obtained with higher phosphorus contents. Forresin cured with 4,4′-aminodiphenyl sulfone (DDS),the OI increased from 22 to 28 when the phosphoruscontent increased from 0 to 1.6 %. For the phenolicnovolac (PN) curing system, the OI increased from 21to 27 when the phosphorus content increased from 0to 2.2 %. A V-0 rating was achieved with both curingagents.

Similar monomers (Scheme 51) were prepared byreacting DOPO with cresol formaldehyde epoxynovolac resin (average functionality, 12).109,110 Theonset degradation temperatures, as measured by TGA,decreased with phosphorus content; however, com-paring with other phosphorus-containing polymers,the decomposition temperatures of these phosphorus-containing epoxies, were relatively high. Char yieldsincreased with phosphorus content. Although incor-porating DOPO into epoxy resins reduced their Tg

Scheme 51.

Scheme 52.

values and crosslink densities, their Tg values were stillhigher than for other phosphorus-containing advancedepoxy resins used in FR-4 circuit board applications.The OI values increased from 23 to 27 when thephosphorus content was increased from 0 to 1.7 %,and increased to 33 when the phosphorus content was3.6 %. The OI for dicyandiamide (DICY)-cured resinwas 34, which was higher than for DDS- or PN-curedresins. This was attributed to the high nitrogen contentin DICY (N = 67 %). A similar trend was observedfor the UL-94 test.

A series of advanced epoxy resins with variousepoxy equivalent weights were synthesized from areactive phosphorus-containing diphenol (Scheme 52)and diglycidyl ether (Scheme 53) and then cured withDDS, PN or DICY.111,112 It was shown that thedecomposition temperatures of the formulations withdifferent phosphorus contents are almost the same asfor non-flame-retardant epoxies. Less than 1 % charyield was found in the non-flame-retardant systemat 700 ◦C under air; however, 10–18 % char yieldswere found in the phosphorus-containing epoxies.The OI increased from 22 to 28 when the phosphoruscontent increased from 0 to 1.4 %; however, it reacheda plateau when the phosphorus contents exceeded2.1 %. In the epoxy formulation for encapsulationof electronic devices, a V-0 rating could be achievedwith a phosphorus content of 1.0 %, comparable to abromine content of 7.2 %.

In another study, the above-mentioned diphenol(Scheme 52) was used as a reactive flame retardant ino-cresol formaldehyde novolac epoxy resin.106,113,114

Because of the rigid, cyclic, side-chain structure ofDOPO, the resultant phosphorus-containing epoxyresin exhibited a higher glass transition temperature,better flame retardancy, higher modulus and greaterthermal stability than the regular bromine-containingtetrabromobisphenol A epoxy resin. A UL 94 V-0

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Scheme 53.

Scheme 54.

rating was achieved with a phosphorus content as lowas 1.1 %, comparable to a bromine content of 6–13 %.

The same diphenol (Scheme 52) and a similarnaphthalene-type material (Scheme 54) have beencopolymerized with novolac-type epoxy resins.115 AV-0 UL 94 rating was achieved in printed circuitboard laminates made with this epoxy at 1.5–2.0 %phosphorus in the resin. A V-0 rating was observedin cresol novolac epoxy formulations if 25 % ofthe phenol-formaldehyde novolac used to cure theepoxy was substituted with the diphenol, dinaph-thol (Schemes 52 and 54) or their related methyl-substituted compounds (Schemes 55 and 56).116

Other flame-retardant epoxy monomers wereprepared by reacting the diphenol (Scheme 52)with DGEBA (Scheme 57) or high-performancenaphthalene-based epoxies (Scheme 58) and com-pared with TBBA (Scheme 32) advanced epoxyresins.117 A very high glass transition temperature

Scheme 55.

Scheme 56.

(Tg = 235 ◦C) was obtained when the tetrafunctionalnaphthalene-containing epoxy resin was used. The OIvalues increased with the increase of bromine con-tent (from 32 to 39 when the bromine content wasincreased from 13.4 to 22.7). Phosphorus was foundto be more effective than bromine in attaining theflame-retardant property (2 % phosphorus seemed tobe as effective as 20 % bromine). A V-0 rating couldbe achieved with 1.4 wt% phosphorus or 13.4 wt%bromine for highly crosslinked resins.

Two phosphorus-containing diacids were synthe-sized from DOPO (Scheme 49) and either maleic acid(Scheme 59) or itaconic acid (Scheme 60) and thenreacted with the diglycidyl ether of bisphenol A to formtwo series of advanced epoxy resins.118 After curingwith DDS, the thermal properties of the cured epoxyresins were studied by using dynamic mechanical anal-ysis (DMA) and TGA, while the flame-retardanciesof the cured epoxy resins were evaluated by using

Scheme 57.

Scheme 58.

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Scheme 59.

Scheme 60.

Scheme 61.

Scheme 62.

Scheme 63.

the UL-94 test. The degradation temperatures werefound to decrease with phosphorus content, whilethe char yield increased with phosphorus content.The itaconic-acid-based formulations required slightlyhigher phosphorus content in order to achieve a ratingin the UL-94 test. For both types of flame-retardantepoxies, incorporating 1.7 % P provided a V-1 rating.

Dicarboxylic acids, containing either aliphatic(Scheme 61) or aromatic (Scheme 62) phosphineoxides, or DOPO-itaconic acid structures (Scheme60), were copolymerized with DGEBA at 22 and30 wt%, respectively.119 When these copolymers werecured with DICY, epoxy resins with a V-0 UL-94rating were obtained.

DOPO (Scheme 49) was reacted with epichloro-hydrin (Scheme 63) and then polymerized to givea linear polyether-type prepolymer (Scheme 64).120

This prepolymer could not be crosslinked when usingamines as hardeners; however, cationic initiators oracid anhydride hardeners were suitable for obtainingepoxy resins with high phosphorus contents and goodflame-retardant properties.

DOPO (Scheme 49) was reacted with the car-bonyl group in 4,4′-dihydroxybenzophenone to givea phosphorus-containing bisphenol (Scheme 65),

Scheme 64.

Scheme 65.

Scheme 66.

which was then reacted with epichlorohydrin orDGEBA to obtain phosphorus-containing epoxymonomers.121–123 Differential scanning calorimetry(DSC) and thermogravimetric analysis revealed thatthese cured epoxy resins had high glass transitiontemperatures and high thermal stabilities. The charyields were found to increase when the phosphoruscontent increased, while increasing the phosphoruscontent resulted in increasing the OI values. TheDICY-cured epoxy resins showed higher OI valueswhen compared to other diamines. This was attributedto a possible phosphorus–nitrogen synergism. Whenthe DOPO-containing epoxy resins were cured witha DOPO-containing diamine (Scheme 66), extremelyhigh OI values, between 37 and 50, were found, result-ing from the high phosphorus contents of the epoxyresins.

Glycidyloxy diphenylphosphine oxide (Scheme 67),a side-chain phosphorus-containing monoepoxide,was mixed with DGEBA and cured with diethylene-tetramine.124 Because of the low crosslink densityand gel fraction of the system with the monoe-poxide, this resulted in poor protection by intu-mescent char during thermal decomposition, andtherefore the OI of the epoxy decreased with anincrease of the monoepoxide, although the phos-phorus content increased. In contrast, the OI ofthe polymer increased with the phosphorus con-tent of the samples when DGEBA was copoly-merized with bis-(3-glycidyloxy)phenylphosphonineoxide (Scheme 12).125,126 These systems showed rel-atively higher char yield and OI values at 29–31,

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Scheme 67.

Scheme 68.

while the epoxy without phosphorus exhibited OIvalues of 19–22. The same resin was copolymer-ized with bis(4-aminophenyl) phenylphosphate andshowed a much higher OI of 34–39.127 Bis-(3-glycidyloxy)phenylphosphonine oxide has also beenused to prepare an interpenetrating network resinin combination with triallylisocyanurate, which wascured with a peroxide.128 The resin containing 2 %phosphorus extinguished immediately after removalof the flame.

Aliphatic diglycidyl phosphonates (Scheme 68),where R is CH3 (Scheme 68a),129,130 C3H7 (Scheme68b)130,131 or C6H6 (Scheme 68c)130 were copolymer-ized with DGEBA and novolac epoxy resins and thencured with bis(4-aminophenyl) ethylphosphine oxide.These adhesives provided V-0 ratings for laminateprinted wiring boards131 or S4/SR2 rating in the DIN551-2 test.130

Phenylphosphonic dichloride was first reacted withtwo moles of a bisphenol and then the dihydroxycompound was converted to an epoxy monomerby reaction with epichlorohydrin (Scheme 69).132

The OI values of these phosphorus-based epoxies(Schemes 69a–69d) ranged from 33.3 to 40.3. Themaximum OI value was obtained for the epoxidewith the highest P content (Scheme 69b), whilethe minimum OI value was exhibited by theepoxide containing the sulfur moiety (Scheme 69d).Chemically bound sulfur was found to show nocontribution to the flame-retardancy of the polymer.

One of these epoxy resins (Scheme 69a) was furtherco-reacted with an s-triazine derivative, thus produc-ing a phosphorus–nitrogen-containing monomer.132

This epoxy was more efficient than the purelyphosphorus-containing resin, hence indicating anN/P synergistic effect. This synergistic effect showeda maximum at a P/N ratio of 1.8. Similarly,epoxy monomers were prepared with phenolph-thalein phosphorus-containing groups (Scheme 70)and s-triazine nitrogen-containing groups. This epoxyshowed a maximum synergistic performance at a P/Nratio of 1.2. It was speculated that steric hindranceof the phenolphthalein group slows down the forma-tion of the P–N network structure responsible for thesynergism.

An epoxy resin containing a cyclic phosphineoxide group in the main chain (Scheme 71)133

(a)�

(b)�

(c)�

(d)�

Scheme 69.

Scheme 70.

Scheme 71.

Scheme 72.

and a tetrafunctional epoxy containing phosphorusoxide and nitrogen groups in the main chain(Scheme 72)134 were synthesized and cured withbis(3-aminophenyl)methylphosphine oxide (BAMP),4,4′-diaminodiphenylmethane (DDM) or 4,4′-di-aminodiphenyl sulfone (DDS), and compared withcommercial resins. In any combination, either curedwith phosphorus-containing BAMP, DDM or DDS,the phosphorus-containing epoxy resins showedhigher char yields than bisphenol A or novolac-type commercial resins. This was extrapolated to thehigh flame-retardant properties of the phosphorus-containing resins.

An oligomer prepared by reacting bisphenol A withphenylphosphonic dichloride (Scheme 73) was usedfor chain extension of DGEBA and compared withcommercial TBBA chain-extended epoxies.135 Epoxyresins with polyphosphonates, containing 1.5–1.8 %

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n

Scheme 73.

CH3

C2H5

C4H9

(a)

(b)

(c)

(d)

Scheme 74.

Scheme 75.

P, exhibited a lower flammability than the analogueswithout P, although the flammability was still higherthan that of the commercial brominated resincontaining 18.5 % Br.

By reacting DGEBA with dialkyl (or aryl) phos-phates, it was possible to chemically modify the epoxyresin (Scheme 74) and then cure it with DDS toobtain a resin with good flame retardancy and ther-mal stability.89 Chemical modification of DGEBA bydialkyl (or aryl) phosphates could be carried out in situduring the curing of epoxy resins without any changein fire behavior. The fire-retardancy of epoxy resinsdid not significantly improve by chemical introductionof dialkyl phosphate grafts (Schemes 74a–74c) at a10 % level. On the other hand, the phenyl phosphatederivatives (Scheme 74d) lead to a good decrease inthe flammability (OI > 28).

Novolac epoxy resins were phosphonylated withphosphonic anhydrates (Schemes 75–77).136,137 Pri-nted circuit board laminates prepared with epoxycontaining 2.7 % phosphorus exhibited a UL 94 V-0 rating. Methylethyl- (Scheme 78) or dimethylpy-rophosphinates (Scheme 79) were used for the phos-phorylation of phenolic novolac epoxy resin, whichensured its flame retardancy.136 Aliphatic pyrophos-phates (Schemes 80–82) or pyrochlorophosphates(Schemes 83 and 84) were made by reacting aliphaticphosphate or chloroalkyl phosphate with P4O10.138

Epoxy resins pre-reacted with 12 wt% pyrophosphateswere cured with a polyamine in order to prepare coat-ings. These coatings passed the DIN 4102 flammabil-ity test.

A phosphorus-containing epoxy resin, bis(3-tert-butyl-4-glycidyloxyphenyl-2,4-di-tert-butylphenyl)resorcinol diphosphate (Scheme 85), was synthesized

Scheme 76.

Scheme 77.

Scheme 78.

Scheme 79.

Scheme 80.

Scheme 81.

Scheme 82.

Scheme 83.

Scheme 84.

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Scheme 85.

Scheme 86.

and subsequently cured with aromatic amines.139 Thephosphorus-free epoxy polymers exhibited OI valuesof 18–23, whereas the phosphorus-containing epoxy(Scheme 85) cured with commercial amines exhib-ited values of 28–31. The larger OI values could beobtained for the phosphorus-containing polymers withhigh aromatic contents. This was attributed to the charcontribution of the phenyl groups.

A multifunctional epoxy resin was preparedby the reaction of epichlorohydrin with (4-diethoxyphosphoryloxyphenoxy)(4-hydroxyphenoxy)cyclotriphosphazene (Scheme 86).140 The epoxy resinwas further cured with diamine curing agents, DDM,DDS, DICY, and 3,4′-oxydianiline (ODA). Com-pared to DGEBA, epoxy polymers (Scheme 86)showed lower weight-loss temperatures, higher charyields, and higher OI values, hence indicating thatthe epoxy resin thus prepared could be useful as aflame-retardant.

Phosphorus-containing curing agentsSimilarly to epoxy monomers, phosphorus-containinggroups can be incorporated into the curingagent. 9,10-dihydro-9-oxa-10-phosphaphenanthrene10-oxide (DOPO) (Scheme 49) is commercially avail-able in Japan and Europe and is recommendedas part of a curing system for halogen-free flame-retardant epoxies.141 Because of the latency of thecuring agent, combination resins offer the easy han-dling of one-component systems. Because the flame-retardant content can be freely varied, the systemscould be adapted to specific flame-retardancy require-ments. Combination resins containing DOPO werefound to meet the UL 94 V-0 rating with phospho-rus contents below 3 %. This meets the require-ments for electrical laminates. However, a studyusing a cone calorimeter showed that a test lam-inate containing this additive still does not meetthe flame-retardancy requirements for aerospaceapplications.

Methylsuccinic anhydride was reacted with DOPOand the product (Scheme 87) was used as a reactive-type flame-retardant with a pendent phosphorus group

Scheme 87.

Scheme 88.

to cure DGEBA and epoxy-novolac.142 Epoxy resinscured with this phosphinate anhydride exhibitedhigher OI values and char yields when comparedwith those cured by commercial anhydrides, such asphthalic anhydride and hexahydrophthalic anhydride.Surprisingly, the phosphinate anhydride showed abetter effect on promoting fire-resistance in DGEBA-based resins than in epoxy novolac-based resins at thesame phosphorus content.

A product of the interaction of DOPO and p-hydroxyphenylmaleimide (Scheme 88) was reactedwith epoxy and cured by phenol-formaldehydenovolac.143,144 The resin thus obtained was usefulfor encapsulating electronic devices and gave V-0 ratings in the presence of a high content ofsilica.

A phosphorus-containing novolac was preparedfrom DOPO and 4-hydroxybenzaldehyde via anaddition reaction (Scheme 89).145 Since this productcontained multi-phenol groups in the molecular chain,it was used as a polyfunctional curing agent. High glasstransition temperatures (above 160 ◦C) and very highthermal stabilities (above 300 ◦C) were observed foro-cresol novolac epoxy cured by using this materials(Scheme 89). The OI values were proportional to thephosphorus contents of the epoxy resins. A minimumphosphorus content of 2 wt% was found to enable thecured epoxy resins to exhibit an OI value of 26 and topass the UL 94 test at V-0.

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Scheme 89.

Scheme 90.

In another study, this phosphorus-containingnovolac (Scheme 89) was compared to the prod-uct of partial phosphorylation of phenol formalde-hyde novolac with diphenyl phosphorochloridate(Scheme 90).146 Owing to the rigid structure ofthe former (Scheme 89), the resultant phosphorus-containing epoxy resin exhibited a higher glasstransition temperature, and thermal stability thanthe phosphorylated phenol formaldehyde novolac(Scheme 90)-containing epoxy or TBBA-containingepoxy resin. By comparing the efficiencies of thesetwo resins (Schemes 89 and 90), it was found that theepoxy resins cured with the phosphorus-containingnovolac (Scheme 89) demonstrated higher OI valuesand shorter burning times in the UL 94 test thanphosphorylated novolac (Scheme 90) (1.35 % P in thephosphorus-containing novalac (Scheme 89) is betterthan 1.71 % P in the partially phosphorylated material(Scheme 90)). These results were in good agreementwith the char yields.

A phosphorus-containing aralkyl novolac was pre-pared from the reaction of DOPO, first with tereph-thaldicarboxaldehyde and subsequently with phenol(Scheme 91).147 This product blended with phe-nol formaldehyde novolac was used as a curingagent for o-cresol formaldehyde novolac epoxy. Theepoxy resins exhibited high glass transition tempera-tures (159–77 ◦C), good thermal stabilities (>320 ◦C)and low thermal degradation rates. High char yieldsand high oxygen index values (26–32.5) wereobserved, hence indicating that the resins had goodflame-retardance. Using a melamine-modified phenolformaldehyde novolac to replace phenol formaldehydenovolac in the curing composition further enhancedthe glass transition temperatures (160–186 ◦C) andOI values (28–33.5) of the cured epoxy resins.

DGEBA was cured with a phosphorus-containingdiamine derived from DOPO and 3-nitrobenzoyl chlo-ride, followed by hydrogenation (Scheme 92).148 This

Scheme 91.

Scheme 92.

Scheme 93.

Scheme 94.

Scheme 95.

resin was compared with the phosphorus-free epox-ies cured with DDS or DDM. The char yield ofthe phosphorylated epoxy at 700 ◦C under nitro-gen was 32 %, whereas the yields of DGEBA/DDSand DGEBA/DDM were 15 % and 13 %, respec-tively. This result was further confirmed by OImeasurements. Compared with the OI values ofDGEBA/DDS (22) and DGEBA/DDM (21), the OIof the phosphorus-containing epoxy was 30. Corre-lations between the high char yields and OI valuesimplied that the flame-retardancy of the epoxy resinswas elevated by using the phosphorus-containingdiamine as a curing agent.

The synthesis of the diamines containing two DOPOgroups (Schemes 93–95) has been described by Chiuet al.123

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Scheme 96.

Scheme 97.

Phosphine oxide structures are often used to impartflame-retardancy to curing agents, because phosphineoxides are thermally and hydrolytically very stable. Acyclic phosphine oxide (Scheme 96) was used in vari-ous epoxy resins, including the naphthalene type andbranched novolac epoxies.115 Most of these compo-sitions showed a V-0 rating in printed circuit boardlaminates at 1.5–2.0 % phosphorus in the epoxy.

A few studies have been reported in theliterature concerning curing of epoxy resins withbis(aminophenyl)methylphosphine oxide (Scheme97), as well as the thermal and combustionperformance of the resins containing thisoxide.56,87,133,149–153 Von Gentzkow et al129 have usedthe analogous bis(aminophenyl)ethylphosphine oxide(Scheme 98) to cure a phosphorus-containing epoxyresin.

Varma and Gupta149 found that glass-fabric-reinforced laminates based on DGEBA and thebis(aminophenyl)methylphosphine oxide (Scheme 97)exhibited a higher limiting oxygen index, as wellas higher shear and flexural strength, than thosebased on the DGEBA/DDS system. Thermal ageingat 185 ◦C for 100 h did not affect the mechanicalproperties of the phosphorus-containing laminates;however, a significant decrease was observed in theinterlaminar shear strength by boiling in water for 100and 200 h. Epoxy resins cured with this oxide (Scheme97) exhibited higher char yields when compared withthose cured by DDS.153 The phosphorus-containingepoxy resins showed self-extinguishing characteristicsin comparison to the DDS-cured sample whichcontinued to burn after removal of the ignition source.The cone calorimetry test showed that the heat-release rate decreased as the phosphorus concentrationincreased.

Shau and Wang133 compared this methylphos-phine oxide (Scheme 97) with 4,4′-diaminodiphenyl-methane (DDM), and DDS in bisphenol Aand novolac epoxy resins, as well as with aphosphorus-containing epoxy with a cyclic phos-phine oxide group (see previous section). Simi-lar comparisons were made with diimide–diepoxidepolymers.56 Epoxy resins cured with the methylphos-phine oxide (Scheme 97) showed higher OI val-ues than non-phosphorus-containing resins; however,

Scheme 98.

Scheme 99.

Scheme 100.

the decomposition temperatures of the phosphorus-containing polymers were significantly lower. Theauthors speculated that phosphoric acid species wereformed upon thermal decomposition, which catalyzedehydration of the polymers at lower temperatures.

Levchik et al150 showed that the fire-retardant effec-tiveness of the methylphosphine oxide (Scheme 97)goes through a maximum with increasing phospho-rus concentration in a tetraglycidylmethylenedipheny-lamine (TGMDA)/DGEBA epoxy blend. This wasattributed to the competition between the char-forming fire-retardant action and promotion of theevolution of combustible gases because of the catal-ysis of degradation. The second action seems toprevail at larger concentrations of the oxide, corre-sponding to 2–2.5 % P. This is also the range ofconcentration where the mechanism of action of theoxide switches from a condensed-phase (charring) toa gas-phase action, probably due to volatilization offlame-inhibiting phosphorus-containing degradationproducts.

In line with the other phosphorus-containing epoxyresins discussed above, it was found that com-pletely aromatic bis(aminophenyl)phenylphosphineoxide (Scheme 99) tends to produce more char thanbis(aminophenyl)methylphosphine oxide (Scheme97).153 It also gives higher char yields compared tobis(aminophenyl)phenylphosphonate (Scheme100).154 Hsiue et al155 studied the flame retardantproperties of this phosphonate in combination withphosphorus- or silicon-containing epoxies and founda phosphorus–silicon synergistic effect.

In another study, the diamine bis(4-aminophenyl)phenylphosphate (Scheme 101) was studied in combi-nation with diglycidyl phenylphosphate39 (see previoussection). High char yields (32–52 %) as well as highoxygen index values (34–49) for these phosphorylatedresins were found. TGA studies showed that the

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Scheme 101.

Scheme 102.

decomposition of the phosphate groups occurredindependently at relatively low temperatures. Thedecomposition of the phosphate groups resulted ina phosphorus-rich residue at the initial stage andslowed down further decomposition of the resins.This not only decreased the decomposition rates inthe high-temperature regions but also resulted in highchar yields.

Various amounts of bis(3-hydroxyphenyl) phenylphosphate (Scheme 102) were added to phenolnovolac as a curing agent for DGEBA.156 Raisingthe phosphorus content of the resin system from 0to 2.4 % increased the char yield from 18 to 35 %.The UL 94 test showed that the flame-retardancyof the cured epoxy resins increased with phosphorusor bromine content (TBBA (Scheme 32)); however,phosphorus was found to be more effective thanbromine as a flame-retardant (1 % phosphorus isbetter than 6 % bromine). In order to achieve a V-0rating in o-cresol formaldehyde novolac epoxy resins,2.2 wt% phosphorus or 12.9 wt% Br was required.114

Phosphorus-containing samples generated much lesssmoke than those containing bromine.

Di(hydroxy-o-tert-butyl)phenyl phenyl phosphate(Scheme 103) was used at 30 wt% loading in DGEBA,which helped to increase the OI from 18.5 to26.5.157

The use of tris(3-aminophenyl)phosphine oxide(Scheme 104) as a curing agent for phosphorylatedepoxy–imide polymer and commercial bisphenol Aand novolac epoxy resins has been reported.55 It was

Scheme 103.

Scheme 104.

Scheme 105.

found that the phosphine oxide increases the charyield for both commercial epoxy resins, whereas thephosphorylated epoxy–imide polymer gives a furtherboost to the char yield.

Tris(hydroxytolylphosphine oxide) (Scheme 105)was prepared by the isomerization of tricresyl phos-phate using an extremely strong base.158 Laminatesprepared with novolac epoxy resin cured with 57 wt%of the phosphine oxide showed a V-0 rating.

A poly(arylene ether surfone) phosphine oxide(Scheme 106), with controlled molecular weightsand amine end groups, was synthesized, and usedas a modifier for DGEBA-based epoxy resins.159

The closely related poly(arylene ether sulfone)(Scheme 107) was utilized for comparison purposes.The epoxy samples modifled with 20 wt% of thephosphorus-containing ether showed a burning timeof 20–30 s after removal from the flame, whereas theepoxy control samples burned up completely.

Scheme 106.

Scheme 107.

1920 Polym Int 53:1901–1929 (2004)


t−C4H9

t−C4H9 t−C4H9

t−C4H9

Scheme 108.

Scheme 109.

Scheme 110.

Two hydroxy-terminated oligomeric phosphates(Schemes 108 and 109) were used alone or incombination with commercial phosphine oxide(Scheme 110) or with the aluminum salt of methylmethylphosphonic acid (Scheme 45).160 Novolac-typeepoxy resins, heavily loaded with silica (82 wt%),required only 1 wt% of the oligomeric phosphate and0.5 wt% of the co-additive in order to ensure a V-0rating.

A series of phenylphosphonic acid amides (Schemes111–113) and an amide of phenyl phosphoric acid(Scheme 114) were evaluated as curing agents forepoxy resins.161 The char yield at 800 ◦C, measured bythermogravimetry, was used as an indicator for flameretardancy of these curing agents. The amide shown inScheme 114 has a lower phosphorus content (4.81 %)than that shown in Scheme 112 (4.93 %); however,the char yield of the epoxy cured with the former(Scheme 114) was significantly higher than that of thepolymer cured with the latter amide (Scheme 112)(30.3 % vs 15.9 %). Moreover, the temperature ofthe beginning of decomposition epoxies cured withthe amide of phenyl phosphoric acid (Scheme 114)was also higher than that for other amide-cured(Scheme 111) epoxies. When companing the epoxiescured with the amide shown in Scheme 113 tothose cured with the amide shown in Scheme 112,it was observed that the former has a higher onsettemperature for thermal decomposition but a lowerchar yield, which indicates that the presence of theadditional aromatic group in this amide (Scheme 113)did not play a significant role in improving the flameretardancy.

Scheme 111.

Scheme 112.

Scheme 113.

Scheme 114.

Scheme 115.

Scheme 116.

Polym Int 53:1901–1929 (2004) 1921

SV Levchik, ED Weil

Scheme 117.

Scheme 118.

Scheme 119.

Scheme 120.

Scheme 121.

Bertram and Davis162 prepared epoxy formula-tions cured by phenol-formaldehyde novolac resinand aromatic or aliphatic phosphoric acid amides(Schemes 115 and 116). These epoxy resins showedimproved flame-retardancy.

Two series of novel phosphorus-containing poly-alkylene amines, with or without aromatic groups,were synthesized via reacting phosphoryl chloridederivatives with commercially available polyether-amines, ethylenediamine and N-phenyl-1,4-phenyl-enediamine (Schemes 117–122).139,163 The phos-phorus-free epoxy polymers were found to have OIvalues of 18–21, whereas the OI values of thephosphorus-containing polymers leveled out at 22–31.A relatively high OI value was obtained for theepoxy cured with the amine shown in Scheme 119,which indicates the role of aromatics in charring.The high phosphorus content also resulted in higherOI values. For example, the polymers shown inScheme 118 (P content = 3.46 %) and Scheme 121(P content = 4.07 %) displayed relatively high OIvalues of 27 and 31, respectively.

Liu et al164 designed and synthesized a highlystable phosphine oxide containing a secondary

Scheme 122.

Scheme 123.

Scheme 124.

diamine, bis(4-cyclohexylaminophenyl) phenyl phos-phine oxide (Scheme 123). The diamine, along withDDS, was then used in the preparation of novel epoxynetworks with improved fire-resistance and tough-ness. The temperature for a 5 % weight loss of themodified epoxy networks occurred at around 400 ◦C.All phosphine-oxide-based epoxy thermosets yielded16.7–17.7 % char at 750 ◦C under air flow, whereasthe control sample, DGEBA/DDS, had no char at thattemperature.

Ito and Miyake143 suggested the use of a commer-cial monofunctional amide (Scheme 124), made byreacting diethylphosphite with acrylamide. This wasefficient in epoxy resins heavily loaded with silica,which have been used for the packaging of electronicdevices.

Phosphorus-containing guanamines (Schemes 125and 126)165 and a spirocyclic bisphosphonate (Scheme

1922 Polym Int 53:1901–1929 (2004)


Scheme 125.

Scheme 126.

Scheme 127.

Scheme 128.

Scheme 129.

Scheme 130.

127)129,166 were used to cure DGEBA resin. Printedcircuit board laminates containing 24 wt% of the gua-namines showed a V-1 rating, whereas those contain-ing 33 wt% of the spirocyclic bisphosphonate displayeda V-0 rating.

The phosphonic acid made by reacting methylphos-phorus dichloride with acrylic acid (Scheme 128),167

or the related anhydride (Scheme 129)168 were usedfor the chain extension of DGEBA epoxy resins. Phos-phonic diacids were also obtained by reacting theanhydride with aliphatic diols (Scheme 130).168 Epoxyresins containing the anhydride or diacids cured withDICY showed a V-0 rating.

Propylphosphonic anhydride (Scheme 131) wasused to cure epoxy resins composed of bisphenolF and bisphenol A diglycidyl ethers and epoxynovolac.169 Hardened plates of 12 layers of glassboards containing 3.5 % P showed good flame-retardant performance in the DIN 5510-2 test.Phosphonic diacids (Schemes 132 and 133) were usedto cure bisphenol F and novolac epoxy resins, whichprovided V-0 ratings, in combination with ATH.170

Scheme 131.

Scheme 132.

Scheme 133.

Scheme 134.

Scheme 135.

Scheme 136.

Scheme 137.

O-methyl P-ethyl-P-methylphosphinate (Scheme134), dimethyl methylphosphonate (Scheme 135),dimethyl ethylphosphonate (Scheme 136) or dimethylpropylphosphonate (Scheme 137) were reacted withP2O5 to make pyrophosphinates or pyrophosphonates,respectively, which were used to cure epoxies.171

The bicyclic phosphate 1-oxo-4-hydroxymethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane (PEPA)(Scheme 138) was combined with DGEBA epoxyresin to obtain halogen-free flame-retardant poly-mers.172,173 These were compared to the resins curedby ethylene diamine or phthalic anhydride. As mea-sured by cone calorimetry, the ignition time increasedand the heat release rate, CO and smoke evolved,decreased with PEPA content. During combustion,the presence of intumescent char was observed, andthe char yield increased with increase of PEPA con-tent. TGA and FTIR spectroscopy results demonstratean increase of crosslinking in the process of ther-mal decomposition, which is partially responsible for

Polym Int 53:1901–1929 (2004) 1923

SV Levchik, ED Weil

Scheme 138.

Scheme 139.

Scheme 140.

Scheme 141.

Scheme 142.

charring. A monofunctional aliphatic phosphine oxide(Scheme 139) was used in combination with ATH toprovide a V-0 rating to cast epoxies.131

La Rosa et al87 studied the flame-retardancyof a dual-cure thermosetting system, consistingof an epoxy resin blended with an unsaturatedpolyester. Halogen-free flame-retarded formulationswere prepared by adding different amounts (5, 10 and15 %) of three commercial phosphorus-containingdiols (Schemes 140–142, respectively). According tothe OI numbers, no significant variation between thedifferent flame-retardants was found.

Silicon-containing epoxy resinsSilica is widely used in molded epoxy formulationsfor encapsulating electronic devices. Because of highloading, it provides a flame-retardant effect mostlydue to the ‘heat-sink’ effect. In contrast, siliconincorporated in the epoxy resin network ensures achemical mode of the fire retardant action, which isobserved at relatively low silicon contents.

Triglycidylphenyl silane (Scheme 143) was synthe-sized and the corresponding silicon-containing epoxy

Scheme 143.

Scheme 144.

resins were prepared by curing with DDS, DICYor DDM.174 Fire-retardant and thermal decompo-sition performances of the silicon-containing epoxywere compared to those of commercial DGEBA epoxyresins.175 The introduction of silicon resulted in highercuring reactivity. Char yields and OI measurementsdemonstrated improved flame-retardancy, because theOI increased from 24 for a standard commercial resin,to 35 for silicon-containing resins.

In another study,155 the same tri-functionalresin was combined with difunctional bis-(3-glycidyloxy)phenylphosphosphine oxide (Scheme 12)and cured either with a siloxane diamine (Scheme 144)or bis(4-aminophenyl) phenylphosphonate (Scheme100) curing agents. An OI enhancement from 26 to36 was observed for epoxy resins containing both phos-phorus and silicon. This synergistic effect was furtherboosted by using siloxane. Epoxy resins consisting ofthe phosphorus epoxide and siloxane diamine exhib-ited a high OI value of 41. While under flame exposure,phosphorus provided a tendency for char forma-tion, silicon provided an enhancement of the thermalstability of the char. Introducing both phosphorusand silicon together in the epoxy resin compositionsuccessfully combines these two factors in a strongflame-retardation mechanism.

New silicon-containing epoxy monomers wereobtained by reacting DGEBA with diphenyl-silanediol (Scheme 145) or o-cresol-formaldehydeepoxy novolac with triphenylsilanol (Scheme 146).176

These resins were cured with DOPO-containingdiamine, triphenylphosphine oxide diamine or DOPO-phosphorylated phenol formaldehyde resin. With sil-icon incorporation, the OI values of the epoxy resinswere increased from 21 to 22.5 and from 21 to 27 forthe silanediol (Scheme 145) and silanol (Scheme 146)systems, respectively. When cured with phosphorus-containing curing agents, the resulting epoxy resins

Scheme 145.

1924 Polym Int 53:1901–1929 (2004)


Scheme 146.

Scheme 147.

Scheme 148.

Scheme 149.

Scheme 150.

showed extremely high OI values of 35 and 49 forthese two epoxy monomers (Schemes 145 and 146),respectively.

A series of siloxane-containing diamines (Schemes147–149) were used to cure a dicyclopentadiene-containing epoxy (Scheme 150).177 Silicon-basedresins exhibited OI values of around 31–34 while thesilicon-free epoxy resin cured with DDM exhibited anOI value below 19.

A silicon-novolac hybrid resin was prepared byreacting polydimethylsilane and phenol-formaldehydenovolac;178 25 wt% of this silicon-containing resinprovided a V-0 rating in bisphenol A epoxy resin.

Various methoxy-phenyl or ethoxy-phenyl polysilox-anes at 10 wt% loadings showed V-0 ratings innovolac epoxy resin cured with phenol-formaldehydenovolac.179 Methyl-phenyl silicone copolymerizedwith some siloxane units showed V-0 ratings in asimilar resin at 9 wt%.180

The diglycidyl phenylphosphonate (Scheme 12)was cured with DDM in the presence of tetraethoxysi-lane (Scheme 151).181 In the presence of the catalyst,ie p-toluenesulfonic acid, tetraethoxysilane decom-posed, which led to epoxy-silica hybrid materials witha nanostructure via an in situ sol–gel process. Theglass transition temperature of the hybrid epoxy resins

Scheme 151.

Scheme 152.

increased with silica content. The OI values of theDGEBA-based epoxy resins increased from 26 to 31,so demonstrating that introduction of silica into theepoxy improved the flame-retardancy. The enhance-ment of the flame retardancy with the increase ofsilicon was not observed in the mechanically blendedsilica–polymer systems; however, it was shown thatnanometer-scale silica could enhance polymer flameretardancy. It was speculated that while being heated,the silicon (in the form of silica) migrated to the sur-face of the material because of the relatively low surfacepotential energy of silicon. The silica on the surfaceof the materials served as a heat barrier to protect theinner layer of the polymers. This protecting effect wasobserved in the TGA thermograms under air, wheresilica greatly inhibited the oxidation weight loss of thepolymeric materials, and resulted in high char yieldsfor the polymers at temperatures higher than 700 ◦C.

Similar organic–inorganic hybrids were preparedby Chiang and Ma182 using tetraethoxysilane anddiethylphosphatopropyltriethoxysilane (Scheme 152).DGEBA was modified by a coupling agent,3-isocyanatopropyltriethoxysilane (Scheme 153), toimprove the compatibility of the organic and inor-ganic phases. The char yield of pure epoxy resin was14.8 wt% while that of the modified epoxy nanocom-posite was 31 wt% at 800 ◦C. Values for the oxygenindex of pure epoxy and the modified epoxy nanocom-posites were 24 and 32, respectively.

Miscellaneous additivesBecause of some difficulties (or at least regulatoryimpediments) with disposal, in particular the inciner-ation of halogen-containing electronic devices, thereis a trend to search for suitable alternatives. Togetherwith phosphorus-containing products, aluminum tri-hydrate (ATH) is finding applications in the flame-retardancy of epoxy resins for printed wiring boards.Various halogen-free formulations are often basedon high-performance epoxy with added ATH. Theseearly halogen-free systems found their niche in themarket place—but not only because of their environ-mentally friendly characteristics. Some halogen-freesystems show a low coefficient of thermal expansionand as a result, are more stable to delamination whencompared to TBBA-containing systems.66

Polym Int 53:1901–1929 (2004) 1925

SV Levchik, ED Weil

Scheme 153.

Scheme 154.

A special grade of ‘high thermally stable’ ATHwas made by heating freshly prepared highly dis-persed ATH at 220 ◦C for 2 h and then treat-ing it with aminoalkylsilane.183 A 70 wt% load-ing of this ATH provided a UL 94 V-0 ratingin the printed circuit board laminates made withDGEBA/DICY.

The effect of ATH and triphenyl phosphate (TPP)on the flame-retardancy and thermal stability ofanhydride-cured epoxy resin was studied by Xiaoet al.86 Surprisingly, it was found that reactive alumina(a Lewis acid) was formed as the ATH lost water,which could catalyze the degradation of the curedepoxy resin. However, the co-operative use of ATHand TPP could lead to higher char yield in spite ofthe catalysis of degradation of the cured epoxy resinand could display condensed-phase flame-retardationsynergism.

It has been illustrated that zinc borate alone canoutperform magnesium hydroxide and zinc carbonatein halogen-free epoxy/novolac systems for electricalapplications.184 Zinc borate could not only providea V-0 performance at 0.8 mm but also improved thethermal stability and copper/epoxy adhesion.

Liu et al147 studied the fire-retardant perfor-mance of cresol-novolac epoxy resins cured withphenol-formaldehyde novolacs and novolac/DOPO-containing copolymers. The epoxy resin exhibited highglass transition temperatures (159–177 ◦C), high charyields and high oxygen index values (26–32.5), henceindicating that the resins had good flame-retardance.Using a melamine-modified phenol formaldehydenovolac (Scheme 154) to replace the regular phe-nol formaldehyde novolac in the curing composi-tion further enhanced the cured epoxy resins glasstransition temperatures (160–186 ◦C) and oxygenindex values (28–33.5). Preparation of the productof condensation of melamine, phenol and resorci-nol bis(diphenyl phosphate) with formaldehyde wasclaimed by Honda et al.185,186 A 28 wt% of the con-densate, used as a curing agent in combination with23 wt% of ATH, provided a V-0 rating for laminatesprepared with a blend (4:1) of DGEBA/cresol novolacepoxy resin.

A similar effect of increasing OI was observed whenmelamine-modified novolac was used to cure silicon-containing epoxies.176 However, char enhancement

was not observed for the melamine-containing resins.It was speculated that under heat, the melaminegroup transformed to melam and melem via adeammoniation reaction. The ammonia gas and theheat-resistant melam and melem could enhance theOI through both gas-phase and condensed-phasemechanisms.

CONCLUSIONSThe thermal stability of cured epoxy resins isstrongly affected by the curing agent used. Thermaldecomposition of any epoxy resin starts from thedehydration of the secondary alcohol in the aliphaticpart of the resin, which leads to the formationof vinylene ethers. The next step of thermaldecomposition is splitting of either the ether or aminebonds, weakened by the allylic groups. Evolution ofhighly combustible small aliphatic fragments follows.Some of the aromatic structures left can undergocharring. In the presence of flame-retardant additives,the thermal decomposition accelerates, although theamount of solid residue increases.

The combustion performance of epoxy resins isstrongly affected by the crosslinking density. Ingeneral, epoxy resins cured with anhydrides are moreflammable than epoxy resins cured with amines. Theoxygen index tends to be positively correlated to thechar yield.

The flame-retardancy of epoxy resins can beimproved by using special epoxy monomers con-taining highly aromatic bisphenols, eg phenolph-thalein, bisphenol-fluorenone, bisphenol-anthrone,tetraphenol-anthracene or bisphenols containing dou-ble bonds able to undergo the Diels–Alder reaction(eg mono-,di- or trihydroxystyrylpyridines). Thesemonomers, cured either alone or in combination with‘regular-grade’ epoxy monomers, show a significantincrease of char yield and oxygen index. Highly aro-matic diamines used as curing agents also help toimprove the flame-retardancy. Some epoxy resins canbe copolymerized with other highly charrable resins,which results in total improvement of the flame-retardancy and physical properties.

For over 30 years, tetrabromobisphenol A (TBBA)has been used in epoxy resins as a co-reactant,especially in electronic grade materials. Industry hasbeen largely satisfied with tetrabromobisphenol A andtherefore very limited activity has been observed inthe development of new halogen-containing flameretardants for electronic applications. On the otherhand, some activity is observed in the compositematerials where high charring tendency and low smokeformation are in high demand.

Phosphorus-containing fire-retardants are partic-ularly effective in epoxy resins because the phos-phate esters or phosphoric acids formed in the ther-mal decomposition of phosphorus-containing prod-ucts tend to react with OH groups and involvethe epoxy resin structure in the charring. Various

1926 Polym Int 53:1901–1929 (2004)


phosphorus-containing additives, eg red phosphorus,ammonium polyphosphate, salts of alkylphosphinicor alkylphosphonic acids, phosphonate esters andphosphate esters, have been shown to be effectiveflame-retardants in epoxy resins. Depending on thestructure of the epoxy resin, curing agent and pres-ence of filler, from 2 to 5 wt% phosphorus is requiredin order to achieve a V-0 rating in the UL-94 test.Phosphorus-containing additives often decrease thephysical properties of the epoxy resin; in particular,phosphate esters tend to decrease the glass transitiontemperature.

There is a relatively large number of publica-tions regarding reactive phosphorus-containing epoxymonomers. Special attention has been paid to the useof 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO). The latter can be either pre-reactedwith the epoxy resin or used as a reactive additiveduring curing of the epoxy resin. Hydroquinone-or naphthalene-type diphenols with a DOPO sub-stituent were used in the chain extension of variousepoxy resins. Because of the highly aromatic struc-ture of these epoxies, a relatively low content ofphosphorus provides a V-0 rating and high OI val-ues. Bis-glycidyloxy aryl- or alkylphosphonates wereextensively studied as comonomers with regular epoxyresins. The phenylphosphonate structure was used asa building block in various epoxy resins because ofthe high efficiency and high thermal stability of thisstructural unit. Aromatic phosphate groups were alsoincorporated into epoxy monomers; however, hin-dered structures were required in order to providesufficient hydrolytic stability.

In a similar way to epoxy monomers, DOPO wasextensively studied as a phosphorus-containing groupin various curing agents. Unmodified DOPO wasrecommended as a part of the curing system forhalogen-free flame-retardant epoxies and provideda UL 94 V-0 rating with phosphorus contentsbelow 3 %. Similar to the phosphorus-containingmonomers, phenylphosphonate or aromatic hinderedphosphate groups were found suitable in curingagents; however, special attention in the literaturehas been paid to triphenylphosphine oxide ordiphenylalkylphosphine oxide structures. Phosphineoxides, in particular, are extremely hydrolytically andthermally stable. Phosphoramides were shown to bereactive with epoxy resins, and therefore have beenconsidered as potential phosphorus-containing curingagents. A combination of phosphorus-containingepoxy monomers with phosphorus-containing curingagents gave a particularly good flame-retardant effectwith an OI as high as 50.

Silicone has been incorporated in both the epoxymonomers and in the curing agents. This was foundto be particularly effective when in combinationwith phosphorus. A melamine-phenol-formaldehydecondensate provided a curing agent rich in nitrogenwhich was shown to be synergistic in phosphorus-containing epoxies.

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