cyanate ester resins polymerization - iowa state...

CYANATE ESTER RESINS

MICHAEL R. KESSLERIowa State University, Ames,

IA

CYANATE ESTER RESINS

The term cyanate ester resin is used to describe both afamily of monomers and oligomers with reactive cyanateend groups (–O–C≡N) on an aromatic ring and the curedresin networks into which the monomers are formed. Thisfamily of thermosetting monomers and oligomers, whichmay alternately be called cyanate resins, cyanic esters,or triazine resins, contain at least two cyanate functionalgroups and will homopolymerize with the addition of heatand/or catalyst into a thermosetting material (a polycya-nurate) used for high performance applications primarilyin the electronic and aerospace industries. These resinshave good processability, shelf life, and compatibility witha variety of reinforcements. Advantages of the cured poly-mer network include high strength and toughness, highglass-transition temperature, low water absorption, lowoutgassing, low dielectric constant and loss, radar trans-parency, and good adhesion to a variety of substrates,while disadvantages include sensitivity to moisture duringprocessing and high costs. Like epoxies, the commerciallyimportant cyanate ester resins are derived from bisphe-nols or polyphenols. Most commercial monomers can berepresented by the structural model illustrated in Fig. 1.

Monomer Synthesis

Grigat and Putter [1] reported the first synthesis ofcyanate ester monomers in 1963 by reacting phenolic com-pounds with cyanogen chloride in the presence of an acidacceptor. They reported near-quantitative yields of sta-ble cyanate esters (with no by-products), which could bedistilled or recrystallized to highly pure materials. Thegeneral reaction for monomer synthesis is shown in Fig. 2.

Alternatively, cyanogen bromide can be used instead ofcyanogen chloride, which, as a solid, is easier to handlesafely [2,3]. This general method, the low temperaturereaction of a cyanogen halide with bisphenol or polyphenolin the presence of a base, is still used to make commercialcyanate esters. After the reaction, which is usually carriedout in solution in the presence of a tertiary amine, thecyanate esters are purified by distillation, recrystalliza-tion, or repeated precipitation [4]. Common side reactionsinclude formation of imidocarbonate when phenol is inexcess or diethyl cyanamide when using cyanogen bromideat temperatures >0◦C. The extreme hazard of handlingand manufacturing with cyanogen halides may be one ofthe reasons that there are only a few companies that arecapable of producing cyanate ester resins on a commercialscale and this explains their relatively high cost.

Adapted from Cyanate Ester Resins, First Edition.

Wiley Encyclopedia of Composites, Second Edition. Edited by Luigi Nicolais and Assunta Borzacchiello.© 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

Polymerization

Oligomers, polymers, and finally cross-linked polymernetworks are formed by heating the monomers througha typical step-growth process. The polymer structure isa network of oxygen-linked triazine rings (cyanurates)and bisphenol ethers. This structure results through thecyclotrimerization of the –O–C≡N functionality. Fullycured cyanate resins are classified as polycyanurates orcross-linked polyarylates. The cyclotrimerization processis shown in Fig. 3. Stable oligomers and polymers can berecovered as liquids, semisolids, hard resins, or solutions.Continued heating of the oligomers yields thermoset plas-tics with cured-state properties equal to those obtainedby uninterrupted heating of the monomer. Cure catalystsand other additives and modifiers are usually compoundedwith the oligomers and polymers before final curing to across-linked matrix. The increase in aromaticity with cure(through the creation of triazine rings) is thought to beresponsible for the unique combination of low viscosi-ties at reasonable temperatures for the monomers, yethigh glass-transition temperature for the cured network[5]—giving cyanate esters the key advantage of epoxy likeprocessing, yet high temperature applicability.

Bauer et al. [6] studied the kinetics of cyanate esterpolymerization and reported an autocatalytic process inwhich phenolic impurities in the aryl dicyanates actedas the polymerization initiator. The investigators [6]explained the polymerization as a multistep processshown in Fig. 4.

1. Initiation by ArOH impurities catalyzes formationof a small quantity of triazine rings.

2. ArOCN reacts with ArOH (catalyzed by existing tri-azine rings) to form carbonic ester imide structures.

3. Carbonic ester imide reacts with two other OCNgroups to form a triazine and an ArOH.

4. The ArOH reacts with ArOCN as in step 2, thuspropagating the cyclotrimerization reaction until allthe ArOCN groups are reacted.

Shimp [7] noted the autocatalytic polymerization effectas long as the triazine polymer retained its mobility (beforegelation). He noted that the apparent restricted mobilityof molecules after gelation makes it difficult to react 100%of the OCN groups (Fig. 5). Note that gelation does notoccur until about >60% of cyanate groups are trimerized,which is higher than the 50% conversion at the gel pointpredicted by the Flory theory for a dicyanate monomer,suggesting the presence of intramolecular cycles in thenetwork structure [8,9].

Without the presence of a catalyst, the trimeriza-tion of cyanate esters is normally a slow process attemperatures around 170–200◦C. In addition, with uncat-alyzed systems, simply increasing the temperature toobtain higher rates may result in significant weight lossesdue to monomer volatility. However, more useful curingrates can be achieved by the addition of specific catalyst

1

2 CYANATE ESTER RESINS

X OO

R

R

R

R

C C NN

Figure 1. General structure for most commercial cyanate estermonomers, where the substituent R and linkage X may vary toimpart specific properties to the resin (Table 1).

systems. Typically, commercial catalyst systems comprisecarboxylate salts and chelates of transition metals suchas copper, zinc, manganese, cobalt, or nickel dissolvedin a hydrogen-donating solvent such as an alkyl phenol.Shimp [7] proposed a trimerization mechanism for thesetransition metal carboxylates–active hydrogen-catalyzedsystems. As illustrated in Fig. 6, he suggests that the metal

acts as a clustering agent, creating a complex to bringthree reactive –OCN groups together, while the activehydrogen source provides reactive carbonic imide function-ality. The coordination metal catalyst is not very effectiveafter gelation, when trimerization proceeds mainly by thestep-growth mechanism shown in Fig. 7.

The heat of reaction for cyanate esters is higher than forepoxy resins, ∼105 kJ/mol for the OCN groups compared to50–58 kJ/mol for epoxies [2]. This can lead to overheatingfrom excessive exothermic reactions in thick compositesand in cases with very fast cure cycles.

Commercial Products

Polymers and oligomers of bisphenol A dicyanate (BADCy)were first offered by Bayer in Europe and Mobay Chem-ical in the United States in 1976 for use as laminat-ing resins for high performance printed wiring boards.

X OH CICN−20 to 20°C

BaseHO

R

R

R

R

X OO

R

R

R

R

C C NN

Figure 2. General reaction scheme for monomer synthesis.

Dicyanate monomer

Hea

t

Heat

Triazine ring

Prepolymer resin

Thermoset plastic(polycyanurate)

X

R

R

ON

R

R

O R′ = X

R

R R

R

NC

NC

N

CO

O

O

X

X

X

OC

N

O

C

N

OC

N R

R

R

R R

R

R

R

RR

RR

O

O

O

R′O

R′O

R′O

OOR′

O

R′

OR′

O

R′

OR'

N

N N

NN

N

NN

N

N

NN

C NC

R′

Figure 3. Curing via cyclotrimerization.

CYANATE ESTER RESINS 3

Ar O C N Ar O C NOHNH

OC

NH

OAr 2Ar O C N N N

N OAr

OAr

O

Ar

OH

+

+ +Figure 4. Polymerization of cyanateester by a small amount of phenol inthe monomer.

0

20

40

60

80

100

0 5 10 15

Hours at 200°C

% c

yana

te tr

imer

ized

20 25 30

Crystalline

Gel

Amorphousprepolymers

128°C HDT

215°CHDT

Thermoset plastic

Figure 5. Isothermal polymerization of bisphenol A dicyanate(uncatalyzed).

Blends of cyanate ester resins and bismaleimides (BMIs)were introduced in 1978 by Mitsubishi Gas Chemical[bismaleimide-triazine (BT) resins]. Cyanate monomercommercialization and research using specialty bisphe-nols has been enabled by Lonza Group AG and sev-eral companies that have changed names and divisionsthrough a series of transaction from Celanese SpecialtyResins to Interez in 1986, to Hi-Tek Polymers in 1988, toRhone-Peulenc in 1989, to Ciba-Geigy in 1992 [3], and then

to Ciba Specialty Chemicals, Vantico, and now HuntsmanCorporation [10].

Today, the largest supplier of cyanate ester monomers isLonza, which sells neat resins, solutions, and prepolymersunder the Primaset™ trade name, followed by Huntsmanwith resins sold under the AroCy trade name togetherwith semisolid and hard resin prepolymers in neat formand as solvent solutions. Some of the monomers/oligomersavailable from these suppliers are listed in Table 1, alongwith properties of the resulting homopolymers. These basicresins from Table 1 are often formulated by various mate-rials suppliers into proprietary systems (often by blendingwith other thermosets or thermoplastics, catalysts, fillers,and/or toughness modifiers).

BADCy, the first commercially available cyanate ester,is currently sold as Primaset BADCy by Lonza and isthe lowest cost and most common cyanate ester monomer.As a crystalline solid with a relatively low melting point,the monomer can be heated to a viscous liquid or usedin solution form—it is soluble in methyl ethyl ketone(MEK), acetone, and other prepreg solvents. The result-ing polymer has a high Tg of 289◦C and a relatively lowdielectric constant, Dk, and dielectric loss, Df . Applicationsinclude radomes and multilayer high speed printed circuitboards. It can be blended with novolac cyanate ester (phe-nolic triazine (PT) resins) and other component to furtherenhance its Tg and to formulate a viscous supercooledliquid at room temperature suitable for liquid composite

M

M

NC

OR

N

C

OR

NC

O R

HOR′

MN

CO

R

N

C

OR

HNC O R

O R′ MN

CO

R

NH

CO

RN

C O R

O R′

MNH

CO

R

NC

OR

NC O R

O R′N

CO

R

N

C

OR

NC O R

+ R′OH +

Figure 6. Trimerization catalyzedby transition metal carboxylates andactive hydrogens.


Figure 7. Trimerization by step-growth mechanism.

O

CN

R′OHO

CN

H

OR′

OCN

O

CN

OR′

OCNH

O

CN

OR′

OCNH

NC

OR

O

CN

OR′

O CN

HNC

OR

OC

N

O

CN

NC O

R + R′OH

Oligomer Oligomer

Oligomer Oligomer

Oligomer Oligomer

Oligomer

Oligomer

Oligomer

Oligomer

Oligomer

molding applications (e.g., as EX-1551-1 resin system fromTenCate Advanced Composites).

Bisphenol E dicyanate (BECy), sold by both Lonzaas Primaset LECy and Huntsman as AroCy L-10, is asupercooled liquid at room temperature with an extremelylow viscosity of 90–120 cP [21], yet it cures to a net-worked polymer with a Tg over 250◦C and a relativelyhigh toughness. As such, this resin is a good candidatefor low temperature processing applications such as resintransfer molding (RTM), filament winding, pultrusion, wetlay-up or injection repair of high temperature composites.It is also often used as a reactive diluent for other cyanateesters to enhance their low temperature processability[22]. For example, BECy has been blended with othercyanate esters to lower the viscosity of the blend such thatit can impregnate fiber filament without heating to enablefilament-wound missile structures [23].

METHYLCy, sold by Lonza, is the only monomer listedin Table 1 with a non-hydrogen substituent (R) group. Theortho-methyl substituents on the monomer repeat unitare responsible for improved hydrophobicity and alka-line hydrolysis compared to the nonmethylated analogs.Blends of METHYLCy with BADCy have also been foundto cure into resins with higher heat distortion temper-atures (HDTs) and wet flexural strengths compared toeither METHYLCy or BADCy homopolymers [5].

Bisphenol M dicyanate (BMCy), sold as AroCy XU 366by Huntsman, has a somewhat modest Tg of only 190◦C,but it has the lowest dielectric constant, Dk, and thelowest dielectric loss, Df , of all of the cyanate ester resins.It also is among the fastest curing, has the lowest moistureuptake (0.7%), and has the highest toughness of the neatresins. AroCy XU 366 has a viscosity of 400–1000 at65◦C. Alternately, BMCy is sold in an oligomeric form asXU-378 with a viscosity of 700–1300 at 82◦C [10]. LikeBECy, BMCy blends well with other cyanate monomersand prepolymers.

Dicyclopentadienylbisphenolcyanate (DCPCy), sold asPrimaset DT-4000 by Lonza, was originally developed byDow Chemical Company as XU 71787.02L and is derived

from phenols and dicyclopentadiene. It has improvedmoisture resistance coupled with excellent mechanicaland electrical properties and processability. The fracturetoughness of this resin can be increased sevenfold byblending with proprietary core-shell rubber (about 0.1 μmin diameter), sold as Primaset DT-7000, without signifi-cantly reducing the resin’s Tg.

PT novolac resins, originally developed by Allied-Signaland now sold by Lonza as Primaset PT-15, PT-30, andPT-60 (which essentially differ in their degree of oligomer-ization, n), are resins with the highest Tg and excellentflame, smoke, and toxicity properties. The PT resins com-bine the processing of epoxies, the Tgs of polyimides, andthe fire resistance of phenolics. High postcuring tempera-tures (>270◦C) are required to reach the high Tg valuesof up to 400◦C. While the PT resins have the highest tem-perature stability, they also have higher moisture uptake(∼3.8%) and higher Dk and Df values than the othercyanate esters (although these values are still low com-pared to other high temperature thermosetting resins).

PRODUCT CHARACTERISTICS

Features

Cyanate ester monomers, resins, and ring-forming poly-cyanurates have these unique properties:

• Monomers are commercially available with >99%assay [24] and ionic impurities <10 ppm.

• Cyclotrimerization rates are essentially dependenton the catalyst used. A variety of transition metalcarboxylates and chelates have been reported[7,25,26], which provide cure response ranging from1 min [reaction injection molding (RIM) class] toshelf-stable prepregs curable at 177–250◦C.

• Toughness/Tg properties, as indicated by impactstrength GIC, strain at break, and adhesive peelstrength, are high for 250◦C Tg resins [27]. Thisunusual combination of properties is attributed

Table 1. Chemical Structure of Commercial Cyanate Esters. Refer to Model Compound:

X OO

R

R

R

R

C C NN

Name (Acronym) Linkage (X) Substituent (R) Trade Name(Supplier)

Physical Stateand Viscosity

Properties of Cured Polymers References

Tg (◦C) GIC (J/m2) Water (%)a Dk and (Df )b

Bisphenol A dicyanate(BADCy)

CH3

CH3

H Primaset™BADCy (Lonza)

Crystalline(MP = 79◦ C)

289 140 2.5 2.91 (0.005) [11–13]

Bisphenol E dicyanate(BECy)

CH3

H

H Primaset LECy(Lonza)AroCyL-10(Huntsman)

Supercooled liquid(MP = 29◦C) 100cP at 25◦C

258 190 2.4 2.98 (0.005) [11–13]

Tetramethyl bisphenolF dicyanate(METHYLCy)

CH2 CH3 PrimasetMETHYLCy(Lonza)

Crystalline(MP = 106◦C)

252 175 1.4 2.75 (0.003) [11–13]

Bisphenol Mdicyanate (BMCy)

CH3

CH3

CH3

CH3

H AroCy XU 366(Huntsman)

Supercooled liquid(MP = 68◦C)8000 cP at 25◦C

192 210 0.7 2.64 (0.001) [11–13]

Dicyclopentadienylbi-sphenolcyanateester (DCPCy)

c H PrimasetDT-4000(Lonza)

Semisolid 1000 cPat 82◦C

265 70 490d 1.4 2.80 (0.003) [11,12, 14–17]

Novolac cyanate esterphenolic triazine(PT)

CH2

OCN

CH2

n

H Primaset PTseries (Lonza)

AroCy XU 371(Huntsman)

Semisolid 20–40cP at 80◦C forPT-15

250,000 cP at 25◦Cfor PT-30

300 to 400 60 3.8 3.08 (0.006) [11,12,18,19]

aWater absorption at saturation (100◦C).bDielectric constant and dielectric loss (tan δ) measured at 1 MHz.cApproximately 20% of the linkages (X) for the DCPCy contain additional dicyclopentadienyl–Ar–OCN segment(s) from the minor mole fraction of oligomeric polyphenols in its precursor [20].dToughened with 10% elastomeric additive as Primaset DT-7000 (Lonza).Abbreviation: MP, melting point.

5


0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

0 20 40 60 80 100

Spe

cific

vol

ume

% conversion

Gel

Measured at 20°C

Measured at 25°C

Figure 8. Volume changes during cyclotrimerization of BADCyat 200◦C.

to the following structural features: ether–oxygen link-ages, low cross-link density, and apparent high free volumein cured state.

• After gelation, the volume increases as conversionincreases above 65% (Fig. 8). This tends to eliminatestress-induced shrinkage at cure temperature.

• Dielectric constant, Dk, (2.6–3.1) and dissipation fac-tor, Df ,(1–6 × 10−3) are unusually low for high Tgresins [28]. High free volume and relatively weakdipoles may contribute to these low loss properties.

• Weight gain due to moisture pickup (1.3–2.4%) inboiling water is lower than epoxy and BMI resins[29]. Long-term stability (>500 h) in 100◦C water hasbeen achieved with epoxy modification or orthome-thylation of the cyanate monomer [30].

• Copolymers with epoxy resins provide hybrid for-mulations with lower cure temperatures, increasedsteam resistance, and lower cost [30,31]. The copoly-mers contain oxazoline rings (cyanate–epoxy resincopolymer), triazine rings (cyanate cyclotrimeriza-tion), and epoxy homopolymer (polyether).

• Several engineering thermoplastic resins have beenused as modifiers for cyanate esters [32]. Low levelsof thermoplastic resin modifiers can be solubilizedin dicyanate; then efficient phase separation can beachieved during the cyclotrimerization step to pro-vide a high degree of toughness [27].

Matrix Properties Versus Epoxy and BMI

Typical cured-state properties associated with cyanateester homopolymers (from difunctional cyanate estermonomers), toughened BMIs, and epoxy resins curedwith aromatic amines are listed in Table 2. Note thatpolycyanurates have the toughness and adhesion of thediglycidyl ether type of epoxies with thermal propertiesintermediate between brittle tetrafunctional epoxies andtoughened BMIs. Advantages of polycyanurates includelower moisture absorption, less cure shrinkage, and lowerdielectric loss properties. In addition, polycyanurateshave superior adhesive strengths up to 250◦C and solventresistance intermediate between difunctional epoxies andtetrafunctional epoxies or BMI resins. This may be due tolower cross-link density of the polycyanurate network.

Blends with Epoxies and BMIs

Cyanate esters can be effectively blended with epoxies,co-reacting to form cost-effective hybrids. While the reac-tion pathway appears to be somewhat complex—involvingcyanate trimerization epoxide insertion and ring cleavagewith additional epoxide to form substituted oxazolidinones[9,33]—the resulting reaction is simplified in Fig. 9.

Likewise, cyanate esters can be copolymerized withBMI resins to create hybrid systems. These so-called BT(bismaleimide-triazine) resins form a high Tg networkstructure through reaction of the double bond of themaleimide group with the cyano groups to form hetero-cyclic six-membered aromatic ring structures with twonitrogen atoms (pyrimidines) as shown in Fig. 10.

Modifying BADCy resins with epoxy or BMI resinscan lead to the properties shown in Table 3. The dataindicate that cure temperatures to obtain >95% conver-sion can be lowered 20–50◦C. The modified systems havelower ultimate Tg values but superior moisture resistance,with no significant loss of physical strength properties.Self-extinguishing flammability ratings are achieved at10–16% bromine content, depending on level of epoxymodification. Epoxy-resin-modified cyanate ester resinsnot only absorb less water than polycyanurate homopoly-mers and tetrafunctional epoxies but also retain a higherpercentage of HDT and reach moisture content equilib-rium sooner. The best performance for long-term boilingwater resistance appears to be formulations with 1.0–1.8epoxide equivalents per monomer cyanate group. Opti-mum cure for epoxy-modified cyanates is offered by copperacetylacetonate with 2 phr nonylphenol catalyst systems.

Resin Forms

Commercial quantities of cyanate ester monomersand resins are available in several forms, includingliquid monomers and oligomers, crystalline monomers,amorphous semisolid prepolymers or oligomers, hardprepolymers in powder or lump form, solvent solutions,and blends. Toxicity screening test for all forms indicatelow health hazards for ingestion or skin contact withbisphenol-type dicyanates. Table 4 gives typical toxicityscreen data. In solution products, the principal hazardsare associated with the solvents. This low toxicity is


Table 2. Matrix Properties Associated with Thermoset Resin Families

Property Epoxya Difunctional CyanateEster

ToughenedBismaleimide

Residual chlorine (ppm) 200–2000 <10 <10Specific gravity 1.2–1.25 1.1–1.35 1.2–1.3Tensile strength (Mpa) 48–90 69–90 35–90Tensile modulus (Gpa) 3.1–3.8 3.1–3.4 3.4–4.1Tensile strain at break

(%)1.5–8 2–5 1.5–3

GICb (J/m2) 70–210 70–210 70–105

Water absorption (%)saturated at 100◦C

2–6 0.07–2.5 4.0–4.5

HDT (◦C)Dry 150–240 230–260 >250Water saturated at 100◦C 100–150 150–200 200–250Coefficient of thermal

expansion (ppm/◦C)60–70 60–70 60–65

TGA onset (◦C) 260–340 400–420 360–400Flammability, UL-94Unmodified rating Burns V-0–Burns V-0–Burns% Br for V-0 18–20 0–12 0–12Dielectric constant at

1 MHz3.8–4.5 2.6–3.0 3.4–3.7

Dissipation factor at1 MHz

2–5 × 10−2 1–5 × 10−3 3–9 × 10−3

Cure temperature (◦C) 150–220 175–250 220–300

aDiepoxides and tetraepoxides cured with aromatic diamines.bDouble-torsion method.

R CH O

CH2

CH2

N

C O R′R CH

O

N

O

R″

Figure 9. Reaction of epoxy group with cyanate group to formoxazolidinone linkage.

R NN

C OR′

OR′

O

O

N

C

R N

O

O

N

N

OR′

OR′ Figure 10. Reaction of maleimide with cyanategroups to form linkage found in BT resins.

in contrast to that of highly irritating and sensitizingproperties of isocyanates, acutely toxic cyanides, andnitriles. Material safety data sheets should be consultedfor specific handling recommendations.

Monomers are available that melt to low viscosity liq-uids (<50 cP) and are useful for formulating high solidssystems and to control viscosity of impregnant systemsfor transfer molding or RIM processing. Formulationscontaining more than 35% crystallizing monomer shouldbe avoided for prepreg usage because crystallization canoccur when the material is stored below the monomer melttemperature.

Prepolymers are partially polymerized (cyclotrimer-ized) amorphous resins consisting of several oligomers and

unreacted monomers. Conversion to trimerized oligomerscan be stopped at the tacky semisolid stage (25–40%conversion; 1000–2000 MW range) or at the hard, grind-able stage (40–55% conversion; 4000–9000 MW range).Semisolid resins are preferred for structural compositesand film adhesives where tack and drape properties arerequired. Hard resins are preferred for compression mold-ing and solution impregnation of woven fabrics such asprepregs for printed circuit boards.

Cyanate monomers and prepolymers are generallyquite soluble in ketone solvent such as MEK and acetone.However, monomer fractions in prepolymer solution cancrystallize out of concentrated solutions.


Table 3. Matrix Properties of BADCy Cyanate Resin Blends

Modification/Cure Blend

25% Br Epoxy 57% Epoxy 57% Epoxy 10% BMI BT 2160

Epoxy resin EpiRez 5163 EpiRez 509 EpiRez 509 —Type Brominated Liquid Liquid —Catalyst (phr)Copper naphthenate 0.24 — — —Copper acetylacetonate — 0.05 0.05 —Zinc octoate — — — 0.02Nonylphenol 2 2 2 —EpiRez 509 — — — 5Maximum cure temperature (◦C) 215 177 200 232

Cured-State PropertiesHDT (◦C)Dry 221 192 196 248Weta 181 178 173 170Water absorptiona (%) 1.65 1.31 1.31 2.02Flexure strength (MPa) 173 117 114 131Flexure modulus (GPa) 3.3 3.1 3.1 3.3Flexural strain (%) 6.6 4.4 4.3 4.4Flammability, UL-94 V-0 Burns Burns Burns

aMoisture conditioned for 64 h at 92◦C and relative humidity exceeding 95%.

Table 4. BADCy Monomer Toxicity Data

Test Result

Acute ToxicityLD50 rat, oral >2500 mg/kgLD50 rat, subcutaneous >2500 mg/kg

Skin Primary IrritationAbraded skin, rabbit NonirritatingIntact skin, rabbit Nonirritating

Skin SensitizationClosed-patch repeated

insult, guinea pigNonsensitizing

MutagenicityAmes screen Negative

Processing

A wide range of methods may be used to process cyanateester composites including wet lay-up, prepreg techniques,filament winding, RTM, compression molding with sheetmolding compound, and pultrusion. This wide range ofprocessing methods is due to the wide range of physicalproperties of the cyanate ester monomers and prepoly-mers [34].

Optimum mechanical properties of cyanate resins areachieved with better than 85% conversion of cyanategroups to triazine structure [27]. Optimum chemical andmoisture resistance properties are achieved at betterthan 95% conversion [7,26,27,29,30]. The references citeddescribe classes of catalysts that can be used to achieveefficient conversion of cyanate monomers and polymers:

• Active Hydrogen Source. This can be an alkylphenol,a bisphenol, an alcohol, an imidazole, an aromaticamine, or a hydroxyl-containing resin; many epoxy

and phenolic resins can be used as the hydroxylsource.

• Soluble Metal Carboxylate Compounds. These can becompounds such as copper naphthenate, zinc octoate,or cobalt acetylacetonate.

Most metal carboxylates have poor solubility in moltencyanate resins, but this problem can be overcome bydissolving the metal compound in a liquid component.Nonylphenol is an excellent solvent for most of the pre-ferred metal catalysts.

Prepolymer solutions for laminating applicationsrequire very low levels of metal carboxylates. Typically,the preferred level is 5–50 ppm of zinc, manganese, cop-per, or cobalt as octoate, naphthenate, or acetylacetonate.The level should be adjusted to provide a varnish geltime of 50–300 s at 177◦C. If there is no epoxy resinin the varnish, 1–2 phr of bisphenol or nonylphenolshould be added as a hydroxyl source. Prepreg dryingtower conditions should be adjusted to volatilize thesolvents with minimal advancement of the resin to highermolecular weight. Some slowly evaporating solvents suchas dimethyl formamide (DMF) may be required to achievethe desired level of fiber wetting and resin flow. Properlyprepared boardy prepregs will have a storage life ofseveral months at room temperatures. Final curing forlaminates is usually done in a press at 177◦C at a pressurethat will provide full wet out of fabric and desired fibervolume.

Unidirectional compliant prepregs for advanced struc-tural composite applications require metal catalyst levelsof 20–200 ppm. The catalyst level should typically beadjusted to provide filming at temperatures that give amatrix viscosity of 0.5–50 Pa and gel times of 2–10 minat 177◦C. These prepregs normally will require storage at0◦C or lower if not converted to composites within a few


weeks. Most structural composites are molded and curedin an autoclave. The following conditions are typicallyused for cyanate-resin–based prepregs:

• Clave pressure should be 345–690 kPa• Vent vacuum bag maintained at 172 kPa• Heat to 177◦C at 1–2◦C/mm• Hold 2–3 h at 177◦C then cool under full pressure to

about 60◦C• Demold and postcure at temperatures up to 250◦C as

required.

Fast-curing formulations (1–15 min) designed for RIM,RTM, or encapsulation of semiconductor devices mayrequire higher catalyst levels. Typical catalysts mightbe 300 ppm Zn or 500 ppm Cu. This level of catalyst canbe achieved by packaging with an epoxy component (RIMand RTM) or by extruder mixing at about 80◦C (moldingpowders).

Processing in the Presence of Moisture

While fully cured cyanate esters exhibit impressive mois-ture resistance compared to epoxies and BMIs, one of thedisadvantages of cyanate ester resins is that care mustbe taken to eliminate the interaction of cyanate esterswith moisture during cure. Even small amounts of mois-ture during the initial stages of polymerization can affectthe chemical structure of the polymer, adversely affectingthe cure and result in depressed mechanical and thermalproperties. Indications that the material has been exposedto moisture during processing include blister formationand microcracking [35].

The problem with moisture exposure during cure is thatthe cyanate monomer is hydrolyzed to form a carbamate;subsequent heating above 180◦C causes the carbamate todecompose, giving rise to an amine and evolving CO2 (asshown in Fig. 11) [36]. The liberation of gaseous CO2 atelevated temperatures is a practical problem in preparinglaminates and is believed to be the cause of blister anddelamination formation at elevated temperatures. Shimpattributed such blisters at the interface in cyanate esterprepreg/aramid-reinforced honeycomb core composites tomoisture evolution and subsequent carbamate and carbondioxide formation during cure and postcure [37]. Likewise,Thunga et al. found that BECy adhesives on BMI/carbonfiber substrates resulted in bubble formation during curefrom absorbed moisture in the BMI resin. However, if the

BMI/carbon fiber substrates were dried before the adhe-sive step, the bubbles were not present in the bond line andthe lap shear strength increase from 4 MPa for the undriedsubstrate to nearly 12 MPa for the dried substrate [38].

In addition, the carbamate structure inhibits thecyclotrimerization reaction and interferes with the poly-mer network development, resulting in lower cross-linkdensity and lower glass-transition temperature. Thiscan be a problem for filled systems, especially when thefillers are strongly hydrophilic. Chao et al. investigatedthe influence of BaTiO3 particles (a strongly hydrophilicceramic) on the properties of BECy and found that readilyadsorbed moisture from the air onto the surface of thepowder resulted in a Tg of just 136◦C compared to 258◦Cwhen the BaTiO3 was first subjected to a rigorous dryingprocess [39]. The type of catalyst is also an importantparameter on hydrolysis and carbamate formation. Forexample, zinc octoate promotes hydrolysis 10 timesfaster than resins catalyzed with Mn+2, Co+2, and Cu+2

carboxylates, and development of catalyst systems thatreduce moisture cure sensitivity is an area of ongoingresearch.

Analytical Techniques

Differential scanning calorimetry (DSC) can be used todetermine monomer purity by examining melting pointdepression and heat of melting. DSC can also be used todetermine heat of polymerization, exothermic peak tem-perature, and cure onset temperature, as shown in Fig 12for BECy resin. DSC has also been used to study kineticsof the cyclotrimerization polymerization reaction [2,40].

Isothermal DSC measurements are used to show howconversion varies with cure time under isothermal curingconditions, as shown in Fig. 13 for a catalyzed BECyresin at cure temperatures of 160, 170, 180, and 200◦C[40]. As can be seen from Fig. 13, the ultimate conversionis limited by the isothermal cure temperature. This is aconsequence of the restricted mobility in the polymer as theglass-transition temperature of the developing networkreaches the isothermal cure temperature. In these cases,a postcure near or above the ultimate Tg of the fully curednetwork is required to achieve compete conversion.

From dynamic DSC experiments following isothermalcures for various times, one can find the relationshipbetween the glass-transition temperature and conversion.An example of such a relationship is plotted in Fig. 14for BECy, where the isothermal cures for various times

R O

C

N

H2O R O

C NH2

>180°CNH2 CO2

O

R ++

Cyanate monomeror oligomer

Carbamate

Catalyst/heat

Figure 11. Hydrolysis reaction of cyanate esters during cure leading to carbamate formation andsubsequent evolution of CO2 at elevated temperature.


−0.1

0

0.1

0.2

0.3

0.4

0.5

200 250 300

Hea

t flo

w, e

xo u

p (w

/g)

Temperature (°C)

259.2°C

259.2°C

844 J/g

Figure 12. DSC of uncatalyzed BECy with heat of reaction,exotherm peak, and onset temperature noted. Heating rate of1◦C/min.

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60

Con

vers

ion

Time (min)

200°C

180°C 170°C

160°C

Figure 13. Conversion versus time for catalyzed BECy resincured isothermally at temperatures ranging from 160 to 200◦C.

at either 180 or 200◦C were used to partially cure thepolymer, and subsequent dynamic DSC experiments wereused to measure the Tg (from the step change in the heatcapacity) and the degree of cure (from the residual heatof reaction) [40]. As seen in Fig. 14, the Tg is shown toincrease monotonically with conversion, with the slopeof the curve increasing as the conversion level increases,and it is independent of the isothermal cure tempera-ture. This relationship is often modeled with the empiricalDiBenedetto equation (Eq. 1) [41], which can be written in

−100

−50

0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1

Isothermal cure at 200°CIsothermal cure at 180°C

Conversion

Tg(

°C)

Tg0

Tg∞

Figure 14. Glass-transition temperature, Tg, versus conversionplot for BECy with DiBenedetto equation fitting the data.

modified form as

Tg − Tg0

Tg∞ − Tg0= λα

1 − (1 − λ)α(1)

In the case of data presented in Fig. 14, λ is found tobe 0.34 ± 0.2 and Tg0 (the glass-transition temperature ofthe monomer) and Tg∞ (the glass-transition temperatureof the fully cured polymer) are found to be -55.9 and276.5◦C, respectively. Since the slope of the Tg versusconversion curve increases with conversion, Tg serves asa more sensitive measure of conversion than residualreaction heat at the latter stages of cure and is often themethod of choice to determine the effect of cure scheduleon the ultimate conversion for various cyanate ester resinsystems.

Prepolymer molecular fraction analysis can be doneusing gel permeation chromatography (GPC) [24]. A typ-ical chromatogram of a low molecular weight oligomerof BADCy is shown in Fig. 15. Note that separation ofunreacted monomer (n = 1) from oligomers (n = 3 andn = 5) and higher molecular weight materials is fairlywell defined.

The cyclotrimerization reaction is frequently followedup with Fourier transform–infrared (FTIR) spectroscopy.The decrease in the strong absorption from the O–C≡Nstretching band in the 2200–2300 cm−1 region can be usedto follow the reaction of the cyanate groups during cure,and triazine formation at 1370 and 1565 cm−1 can alsobe used to study the kinetics of the polycyclotrimerizationreaction [30]. Typical results for a BADCy resin are shownin Fig. 16, where the cure of a solvent cast film of BADCyis monitored from 25 to 200◦C at 20◦C/min [42]. Noticethe disappearance of the absorption peak doublet at 2269and 2232 cm−1 with cure, and the corresponding increasein the absorbance bands of the triazine ring at 1370 and1565 cm−1. The results correlate well with the kineticsdata obtained using DSC [6]. Copolymerization of cyanate


36%

12%

21%

31%

n7 and up

n 5

n 3

n 1 347

776

14152199

MwMnMw/Mn

26827403.6

Figure 15. Prepolymer molecular fraction analysis by gel per-meation chromatography.

Abs

orba

nce

(a.u

.)

Wavenumber (cm−1)

2500 2000 1500 1000

25°C

100°C

175°C

200°C

Doublet from -OCNfunctional group

Triazine rings at1565 and 1370 cm−1

Figure 16. FTIR spectroscopy of a curing BADCy polymer heatedfrom 25 to 200◦C at 20◦C/min.

esters with epoxy resins can also be followed using IR andFTIR spectroscopy[30].

Thermal stability of cured polycyanurates can be deter-mined using thermogravimetric analysis (TGA). TypicalTGA curves for homopolymer cyanate esters, BMIs, anda tetrafunctional epoxy (tetradiglycidyl diaminodiphenyl-methane (TGMDA) cured with diaminodiophenylsulfone(DDS) hardener) are compared in Fig. 17 [29].

Dynamic mechanical analysis (DMA) can be used tostudy mechanical properties such as loss modulus (E′),tan δ, and Tg. An interesting application of DMA isto determine the degree of phase formation in cyanateester/thermoplastic resin blends and alloys. This is illus-trated in Fig. 18 [27], where the addition of curing catalystenhances the formation of a two-phase matrix at lower geltemperatures.

0 200 400 600 800

Temperature (°C)

BADCy

METHYLCy

BMI–MDA

BMI–DAB

TGMDA–DDS

411

403

369

371

306

Rel

ativ

e w

eigh

t (%

)

20%

Figure 17. Thermogravimetric analysis in air.

APPLICATIONS

Printed Circuit Boards

The low dielectric constants and low dissipation factorsthat can be achieved with polycyanurate matriceshave accelerated commercial interest in the electronicsindustry. Major applications are for obtaining fastersignal speed and strength in computer circuitry and formicrowave transparency in radar and communicationhousings. The first commercial end use for cyanate resins,and the largest, is for printed circuit boards [28].Cyanateresins are preferred over epoxy resins for use in highreliability multilayer boards for applications such ascomputer circuitry because of the following performanceadvantages:

Dimensional Stability. Multilayer circuit patternsinterconnected between layers are prone to failuredue to differential movement caused by dissimilarcoefficients of thermal expansion (CTEs) andswelling by chemicals such as solvents, etchants,and strippers. Resins with high Tg approachingmolten solder temperature (250◦C), have lessthermally induced stress because CTE values belowthe Tg are lower by a factor of 2–5 times the CTEabove the Tg.

Corrosion Resistance. Ionizable resin impurities such asepoxy chlorohydrin residues, can attack metal con-ductors in hot–moist environments. Cyanate ester


288°C

0 50 100 150 200 250 300 350

Loss

mod

ulus

(E

″)

Temperature (°C)

198°C

268°C

206°C

No catalyst177°C gel

2.0% strain

Catalyzed140°C gel

5.8% strain

Figure 18. Thermosetting–thermoplastic phase development in80/20 BADCy-polyarylate alloys.

resins, which are largely free from ionizable impuri-ties, are not as susceptible to filamentary growth ofcopper at anodic sites.

Low Dielectric Loss Factors. Low dielectric constantand dissipation factor increase signal speed andsignal strength, particularly in high frequency,elevated-temperature service. See Table 5 for dataon selected fiberglass laminates used for printedwiring boards.

Field Repair. Manual soldering of printed circuit boardsoften raises temperatures of laminates to 300–350◦Cfor short periods. Decomposition of the resin matrixunder these conditions can cause blistering and lossof adhesion. The high Tg and thermal stability ofcured cyanate resins minimizes this type of failureduring repair.

Similarly, cyanate ester resins are used in microelec-tronic packaging, especially for devices with high clockspeeds, where low dielectric loss properties are advanta-geous since they permit increased signal speed and circuitdensity with lower power requirements and heat genera-tion. Moisture resistance of cured cyanate resins has beenimproved by development of high purity monomer (>99%),development of superior cure catalysts [26], and develop-ment of more hydrophobic cyanate esters. Modificationof cyanate esters with BMI resin also improves moistureresistance and thermal stability [43].

Radomes

The low dielectric loss and permittivity of cyanate esterscombined with excellent thermomechanical propertieshave made cyanate esters the preferred resin for radomesand nose cones on aircraft and high frequency applica-tions (such as weather radar), because they result inlower reflectance, increased range or signal strength,and improved signal quality. In some applications,cyanate ester composites are reinforced with quartz orhigh modulus polyethylene fibers to provide improvedmicrowave transparency over fiberglass-based composites.One example is Cytec’s 5575-2 4581 Astroquartz prepreg,which results in a composite (35% resin content) with atemperature stable Dk of 3.1, an out-of-plane Df of 0.005,and an in-plane Df of 0.004.

Structural Composites

Cyanate esters reinforced with carbon fibers are thematerials of choice for structural composites used inspace applications and have replaced epoxy-resin-basedcomposites in these applications because of their lowermoisture absorption, which results in reduced outgassingand greater dimensional stability in a space environment,higher fracture toughness, higher glass-transition temper-atures, lower cure shrinkage, and resistance to ionizingradiation.

In addition to space applications, research anddevelopment activities have increased in the area ofcyanate-ester-based structural composites to take advan-tage of the unique properties of these materials, whichinclude the following:

• Prepolymers have hot-melt processibility combinedwith superior tack- and drape-retention properties.

• Latent metal catalysts are reported that providethree weeks of prepreg stability at 25◦C combinedwith the ability to cure by vacuum bag/autoclave at177◦C followed by postcure at 210–250◦C [26,27,29].

• Dicyanate homopolymers when fully cured have240–270◦C Tg and higher fracture toughness andstrain at break than either epoxies or BIMs [29](Table 2).

• High Tg thermoplastics may be used to modifycyanate ester systems to improve fracture toughness.It has been reported that 15–20% thermoplasticmodification can increase fracture toughness byfive times via an efficient phase separation duringcuring [29]. Fig. 19 compares the effects of severalthermoplastic resins on fracture toughness. Manythermoplastic resins are readily soluble in dicyanatemonomers, making this modification relatively easyto achieve.

Adhesives

Polycyanurates have excellent adhesion to variouslaminates and are finding increased use for high valueadded commercial adhesive applications. Shear strengthsof 27–48 MPa have been reported [44] for aluminum,


Table 5. Properties of Fiberglass Printed Wiring Board Laminates

Properties Unmodified Modified

BADCy METHYLCy DCPDCy BT 2170 BADCy

CompositionModification — — — Epoxy EpoxyModifier (wt%) 0 0 0 45 60Bromine (%) 0 0 0 12 16Glass reinforcement 7628a 7628a NA NA 7628Resin content (wt%) 40 ± 1 40 ± 1 NA NA 40 ± 1

Cure TemperaturesPlaten (◦C) 177 177 177 175 177Postcure (◦C) 232 232 225 NA None

Laminate PropertiesTg (◦C) 255 247 255 210 182TGA onset (◦C) 405 427 425 305 300Flexure strength (MN/mm2) 448 414 NA 483 483Peal strength (N/cm)At 25◦C 21.0 20.6 15.8 16.7 17.2At 100◦C 19.1 18.6 12.3 13.7 16.6At 200◦C 15.8 16.7 10.5 11.8 14.9Steam/solder float (min)b 90 90 NA NA 120Dielectric constant at 1 MHz 4.1 3.8 4.0 4.2 4.2Dissipation factor at 1 MHz 0.004 0.003 0.003 0.008 0.011Flammability, UL-94 Burns V-1 Burns V-0 V-0

Abbreviation: NA, not applicable.a7628 Style glass cloth with Z-6040 finish.bMinutes of conditioning in 121◦C steam passing 20-s float in 260◦C solder.

titanium, and ferrous metals up to 235◦C. These highadhesive strengths occur despite the absence of strongpolar groups in the cured polymer and are attributedto the relatively large failure strains and the potential

100

300

500

700

900

0 5 10 15 20

GIC

(J/

m2 )

% Thermoplastic

CPE

PSPEIPES

PAr

TGMDA/DDS

Figure 19. Fracture toughness response to thermoplastic resinconcentrations in BADCy alloys: copolyester (CPE), polysulfone(PS), polyetherimide (PEI), polyethersulfone (PES), polyarylate(Par), tetrafunctional epoxy reference compound (TGMDA/DDS).

for forming covalent bonds with substrate hydroxylgroups and/or coordination with metal oxides to promoteadhesion [37]. Adhesive applications include structuraladhesives for supersonic aircraft and die-attach adhe-sives. Other non-fiber-reinforced applications for cyanateester resin include motor windings and low frictionbearings.

Flame Retardant Applications

Novolac cyanate esters have the highest thermal perfor-mance of the commercial resins and also provide inherentflame retardancy with very low smoke emissions. Onegrowing application for such resins is in aircraft cab-ins, which have strict limits for smoke/fire/toxicity. Thenovolac cyanate esters exceed these limits and eliminatethe release of volatiles during cure in the phenolic resinspreviously used in such applications. For these reasons,composites from cyanate ester resins have been exten-sively incorporated into the aircraft cabins of the newestBoeing and Airbus airplanes.

One resin of increasing interest for flame retardantapplications is the experimental bisphenol-C cyanateester, which is derived from 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene. Through extensive characteriza-tion by Lyon et al. at the Federal Aviation Administration,this resin, which has mechanical and thermal propertiesequivalent to its bisphenol A analog, is ignition resistantand has a very low heat release rate in forced flaming com-bustion. It has been considered for structural compositesused on US Navy submarines.

An additional resin that is used as a low smoke, non-halogenated flame retardant for cyanate esters (as well as


with epoxy and BMI resins) is Lonza’s Primaset FR-300.This phosphorous-containing difunctional cyanate estercan be blended with other resins at levels of 10–20%to obtain a UL 94V-0 flammability rating. A 10% load-ing of FR-300 with PT-15 novolac resin changes therating from flammable to UL 94V-0. The advantage ofnonhalogenated approach is important in high perfor-mance semiconductor applications and in mass transporta-tion where stringent demands are placed on low smoketoxicity.

FUTURE APPLICATIONS AND OUTLOOK

Over the last several decades, cyanate ester resins havegained acceptance as a replacement for epoxies in highperformance composites, especially in the electronics andaerospace fields, because of their attractive physical,mechanical, and electrical properties. As capacity ofcyanate esters increases, the prohibitively higher costs forcyanate esters are expected to decrease and open up newapplications. While the introduction of new cyanate estermonomers has stagnated over the last decade, the devel-opment of formulations, blends, and copolymers usingthese basic monomers is expanding as new applicationsand property requirements arise. Emerging applicationsfor cyanate ester resins include optical devices, highperformance powder coatings, composites for γ - andneutron-radiation resistance, high end friction materials,and high temperature tooling and casting materials.

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FURTHER READING

Hamerton I. Chemistry and technology of cyanate ester resins,Glasgow and London: Blackie Academic and Professional;1994.

Nair CPR, Mathew D, Ninan KN. Adv Polym Sci 2001;155:1–99.

cyanate ester resins polymerization - iowa state...

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