11. Cyanate Esters - pdf.xuebalib.com:1262

11 Cyanate Esters

Andreas Kandelbauer

School of Applied Chemistry, Reutlingen University, Germany

O U T L I N E

Introduction 425

Chemistry 427

Characterization and Reaction Monitoring 429

Processing of Cyanate Ester Resins 430

Properties of Cyanate Esters 431

Properties of Cyanate Ester Blends

with Epoxy and BMI 435

Recent Developments 437

Applications 448

Trade Names 451

References 451

Introduction

Cyanate ester resins are an important class of

thermosetting compounds that have experienced

an ever-increasing interest as matrix systems for

advanced polymer composite materials, which

among other applications, are especially suitable for

highly demanding functions in the aerospace or

microelectronics industries. Other names for cyanate

ester resins are cyanate resins, cyanic esters, or tri-

azine resins. The various types of cyanate ester

monomers share the aOCN functional group that

trimerizes in the course of resin formation to yield

a highly branched heterocyclic polymeric network

based on the substituted triazine core structure. The

basic reaction sequence leading to the typical cyanate

ester polymer molecule is depicted in Figure 11.1.

The curing reaction may take place with or without

catalyst.

Cyanate esters display a unique property profile.

This includes high glass transition temperature,

low dielectric loss/dissipation factor, radar transpar-

ency, low moisture absorption, low outgassing,

good adhesion to metals and other substrates, and

relatively high strength and high fracture toughness.

Due to the high aromatic content they also show

inherently good flame retardancy and rather low

smoke generation. Furthermore, cyanate esters

usually have good processability since the dicya-

nate monomers melt at low temperatures and are

available in a wide range of physical forms (liquids,

low melting solids, prepolymers of different molec-

ular weights), long shelf lives, and good compati-

bility with a variety of reinforcing materials. The

monomers are of exceptionally low toxicity and

no volatile compounds are released during thermal

curing. They show good compatibility with numer-

ous other thermosetting materials and, hence, have

frequently been employed to modify phenol and

epoxy resins or for blends and copolymers with bis-

maleimide resins [1] to, for example, modify their

brittleness. Some major distinctive features of cya-

nate ester monomers are summarized in Table 11.1.

Due to their attractive properties, it does not

come as a surprise that cyanate esters have increas-

ingly been investigated during the past years.

Numerous novel composite systems have been

developed especially for high-performance matrix

materials in the aviation, aerospace, and electronics

industries. Because of their outstanding thermo-

mechanical and interesting dielectric properties,

they are discussed as major future candidates for

applications in the electronics sector with the

potential to substitute the currently used materials

(mainly epoxies) to a great extent. Important appli-

cations range from structural composites in aircrafts

425Handbook of Thermoset Plastics. DOI: http://dx.doi.org/10.1016/B978-1-4557-3107-7.00011-7

© 2014 Elsevier Inc. All rights reserved.

http://dx.doi.org/10.1016/B978-1-4557-3107-7.00011-7

or satellites to products under high temperature and

mechanical wear conditions like gas turbines, etc.,

to printed circuit boards and other electronic parts.

The trend towards increasing industrial demand

of cyanate ester-based materials is reflected by a

steadily increasing number of scientific studies that

have been published on these thermosets during the

past years (Figure 11.2). The published data on

cyanate ester resins can be roughly categorized into

four main topics: (1) new monomers for novel cya-

nate esters, (2) blends of cyanate ester resins with

other polymeric systems, (3) copolymerization of

cyanate ester monomers with other types of mono-

mers to yield novel high-performance thermosetting

systems, and (4) different kinds of composites of

cyanate esters containing micro- and nanoscale

reinforcements based on particulate, layered, or

fibrous reinforcing materials.

Table 11.1 Distinctive Features of the CyanateEster Monomers [2]

StructuralFeature

Property

Cyanatefunctionality

Low toxicity

Easy to process

Reacts with epoxies

Blends with thermoplastics

Ring forming High service temperatures

aOalinkages Tough

Low cross-linkdensity

Tough

Low polarity Low dielectric loss

Low moisture absorption

High purity Low dielectric loss

Low corrosion potential

N

O

O

O

O

OO

O O

C O O C O

O

O

O

O

O

O

C

O

O

O O

O

O

O

O

O O O

O

O

O

O

O

C

O

OX

XX

XX

X

X

XX

X

XX

X

X X

X

XX X

N

NN

N

N

N

N N

N

N

N

N

N

N N

N

N

N

N

N N

N N N

N

N

N

N

N N

R

R

N

N N N

N

N

N

N

N N

N

Figure 11.1 Basic reaction scheme illustrating the formation of cyanate ester resins. Difunctional monomers

containing two cyanate ester groups react via addition polymerization to yield highly branched polymers

containing aromatic heterocyclic nuclei. The cyclo-trimerization reaction of three cyanate ester functionalities

to yield a triazine ring is highlighted in red in the reaction scheme. The monomers (highlighted in blue) may

contain a wide variety of linkage groups X connecting theaOCN functionalities.

180

160

140

120

100

80

60

40

20

02000 2005 2010 20151995

Num

ber

of s

cien

tific

pub

licat

ions

Publication year

Figure 11.2 Number of scientific publications found

in the scientific database Scopus from 1996 to 2013

using “triazine” or “cyanic ester” or “cyanate ester

resin” as the search terms.

426 HANDBOOK OF THERMOSET PLASTICS

Numerous composite materials based on cya-

nate esters have been recently described in the litera-

ture where cyanate esters have been combined,

blended, and compounded with a wide range of

other thermosetting materials such as epoxy [3�5]

or bismaleimide resins [6�8], thermoplastics, rub-

bers, and fillers, modifiers, and reinforcements.

Reinforcements include a wide range of organic and

inorganic materials and nanomaterials such as alumi-

num phosphate [9,10], aluminum nitride [11�13],

aluminum borate whiskers [14], ceramics like

CaCu3Ti4O12[15] or zirconium tungstate [16,17],

expanded graphite [18], graphene [19], graphene

oxide [20], carbon nanotubes [13,18,19,21,22], car-

bon fibers [7,8], polyacrylnitrile fibers [23], glass

fibers [24], glass microspheres [25], alumina nano-

particles [26], silicon nanoparticles [27], sol-gel

derived [28] or fumed [29] silica nanoparticles, hol-

low silica tubes [30], fumed nanoclays [20,31,32],

silsesquioxane/POSS [33,34], or bentonite [35,36].

Cyanate ester resins have been thoroughly cov-

ered by the seminal book edited by Hamerton

(1994) [37,38], which still represents an important

reference work for the basic chemistry and technol-

ogy of cyanate ester resins. More recently, several

excellent reviews have been published that cover

the scientific and technological progress during the

last decade in this rapidly evolving field of high-

performance thermosetting polymers, for example,

[39,40]. A huge number of scientific reports have

been filed that document the progress in the appli-

cation of cyanate ester/composite systems. To cover

all new developments in this broad field would take

a complete book chapter in itself. However, due to

time and space restrictions, this chapter will only

feature a brief outline of the most important aspects

of cyanate ester resin chemistry with respect to

their main applications, and only some of the most

recent developments directly related to cyanate

ester resins and their monomers will be highlighted.

For a deeper coverage of the topic, especially

regarding the wealth of information on blends,

copolymers, and composites of cyanate esters, the

reader is referred to the above-mentioned mono-

graphs and the original literature.

Chemistry

Cyanate ester resin monomers can be prepared

from various difunctional phenolic starting materials

such as, for instance, bisphenol A or similar com-

pounds by reaction with cyanogen chloride or cyan-

ogen bromide to introduce the cyanate functionality

into the aromatic core. The general basic structural

unit defining a cyanate ester is shown in

Figure 11.3. Typically, R consists of a rigid, aro-

matic structure conferring good mechanical and

thermal properties. The aromatic cyanates are then

trimerized and cured under formation of a three-

dimensional thermosetting network as illustrated in

Figure 11.1 to yield highly branched structures con-

taining substituted triazine rings.

One major advantage that has significantly pro-

pelled cyanate ester research is the relatively low

viscosity of the typical cyanate ester monomer

molecules that, hence, can comparatively easily be

handled. The heterocyclic aromatic structure that

is mainly responsible for the interesting thermo-

mechanical performance is formed only upon poly-

merization during the final cross-linking and is

not yet present in the uncondensed state. This is

often not the case with many of the other currently

used high-temperature, stable thermosetting materi-

als, which makes cyanate ester monomers more

suitable from an application point of view. Also, the

pre-polymers usually have favorable physical prop-

erties in terms of solubility or low melting points.

The chemical structures of some important

monomers used for industrial cyanate ester resin

preparations are summarized in Table 11.2. Various

types of bisphenols with different substitution pat-

terns are typical raw materials, such as bisphenol

A, bisphenol E, bisphenol C, tetramethylbisphenol

F, and bisphenol M. Bisphenol A containing per-

fluorinated methyl groups at the bridging carbon

atom (hexafluorobisphenol A) is also used. Besides

the bisphenol-based monomers, other important

monomers are based on phenolic resin-type struc-

tures like novolacs carrying OCN-groups.

The curing of cyanate esters can be brought

about in the presence or absence of catalysts.

Typical curing temperatures are in the range of 170

to 200°C. Typical commercial catalysts used for

enhancing polymerization of cyanate esters are

N

RO O

C

C

N

Figure 11.3 Basic structural unit of cyanate esters.

42711: CYANATE ESTERS

Table 11.2 Cyanate Ester Monomers [41]

Monomer Structure Material/CAS Registry Number Trade Name Supplier MolecularWeight (g/mol)

NCO OCN

CH3

C

CH3

CH3

C

CH3

Bisphenol M cyanate ester

[127667�44�1]

Arocy XU-366 Huntsman,former Ciba

396,49

OCN Dicyclopentadienylbisphenol cyanate ester

[135507�71�0]

Arocy XU-71787 Dow 251,32

OCNNCO

CH3

C

CH3

Bisphenol A cyanate ester

[1156�51�0]

Arocy B-10 Huntsman,former Ciba

278,31

OCNNCO CH2

CH3

CH3

H3C

H3C

Tetramethylbisphenol F cyanate ester

[101657�77�6]

Arocy M-10 Huntsman,former Ciba

306,36

OCNNCO

CH3

C

H

Bisphenol E cyanate ester

[47073�92�7]

Arocy L-10 Huntsman,former Ciba

264,28

CF3

CF3

OCNNCO CHexafluorobisphenol A cyanate ester

[32728�27�1]

Arocy F-10 Huntsman,former Ciba

386,25

CCl Cl

OCNNCO C

Bisphenol C cyanate ester

[not assigned]

RD98-228 Huntsman,former Ciba

331,16

CH2CH2

OCNOCN OCN Phenol novolac cyanate ester

[30944�92�4]

Arocy XU-371 Huntsman,former Ciba

381,39

CH2CH2

OCNOCN OCN Phenol novolac cyanate ester

[173452�35�2]

Primaset PT-30 Lonza 381,39

metal ions such as Cu21, Zn21, Mn21, Co21, or

Ni21 as carboxylates or chelate complexes dis-

solved in a solvent. Depending on the catalyst used,

a wide range of processing windows can be real-

ized, ranging from rapid cure responses of ca. 1

minute for reaction injection molding applications

to long-term stable pre-pregs that can be stored

until further processing in, for instance, molding

presses at temperatures above 177�250°C [39].

Characterization and ReactionMonitoring

Optical Spectroscopy

Besides structural characterization of the materi-

als by methods such as nuclear magnetic resonance

spectroscopy, optical spectroscopy has widely been

used to monitor the reactions involved in cyanate

ester formation, cure, and thermal degradation

[41�46].

For infrared spectroscopic studies, measurement

set ups in attenuated total reflectance mode have

been widely employed, either by employing off-

line devices, where sequential measurements are

performed at different reaction times after remov-

ing small samples at regular intervals from the

bulk reaction mixture and depositing a drop on the

ATR crystal, by employing heatable micro-ATR

stages onto which the reaction mixture is deposited

and that allow curing monitoring in real-time, or

by employing in-line measurements, where an

ATR sensor probe is immersed into the reaction

mixture and infrared spectra are recorded on a con-

tinuous basis.

Since polymerization of cyanate esters primarily

takes place via cycloaddition of three monomers

under formation of a triazine ring, distinct changes in

the infrared spectrum are to be expected that can be

used for IR on-line monitoring of the curing reaction.

Two strong characteristic bands grow during triazine

ring formation, the ϑ(CQN) at 1553�1565 and

the ϑ(OaCQN) at 1354�1365 cm21, depending on

the exact matrix environment [45]. Cyanate conver-

sion is observed by the decrease in absorbance of the

CRN stretch mode at 2260/2233 cm21 [45]. For

quantification, as a stable reference peak not affected

by the polymerization reaction, the aromatic ring

out-of-plane bending mode appearing at 1013 cm21

can be used [45].

Naturally, besides formation of the triazine ring

that is observed with every kind of cyanate ester,

other typical bond formation reactions that take

place when copolymerization with other monomers

or oligomers is performed can also be followed by

infrared. For example, when cyanate esters are

covalently reacted with epoxies, new oxazoline

functionalities (aNQCaOa) and ether linkages

(aNQCaOaAra) are formed that result in

increasing absorbances at around 1680 cm21 and

1295 cm21, respectively [44].

In-line fiber optical Raman spectroscopy was

shown to be an effective real-time monitoring tool,

for example, when a bisphenol E dicyanate ester

resin was prepared from Arocy L-10 [42]. In a uni-

variate approach, the authors monitored the reaction

progress by using indicative changes at specific

bands in the Raman spectrum: the shifting of a

peak at 801 cm21 into a new mode at 854 cm21

with a low- and a high-energy shoulder (due to

branching at the alpha carbon as the first step in the

cyclization reaction) and the splitting of the ether

linkage mode at 1193 cm21 into two modes at

1172 and 1203 cm21. During curing, the relatively

weak signals from the symmetric and asymmetric

CN modes at 2239 and 2269 cm21 decrease to zero

while the formation of the triazine ring is recog-

nized by a growing peak at 986 cm21, which

is characteristic for the symmetric triazine ring

breathing mode [42]. Although the univariate (sin-

gle peak-focused) analysis gave instructive results,

multivariate data analysis (see Chapter 18), which

utilizes the full spectrum as an information source,

was preferred by the authors in determining the

cure percentage.

Moreover, with their in-line optical fiber setup,

the authors monitored the local sample temperature

during curing by comparing the intensities of the

(temperature-independent) Stokes and the (tempera-

ture-dependent) anti-Stokes Raman peaks originat-

ing from the same intense mode, well resolved and

unaffected by the curing reaction at 633 cm21 (and,

respectively, �633 cm21) resulting from the CaH

out of plane deformation of a phenyl ring.

Thermal Methods

The curing kinetics of cyanate esters is also

typically studied and modeled using thermal and

viscosimetric methods. DSC and TGA are used

here as the typical methods to characterize glass


transition temperatures and thermal degradation as

well as conversion degree upon curing (see below).

Thermal analysis (dynamic differential scanning

calorimetry) in combination with isoconversional

kinetic methods as described in Chapter 18 were also

applied [47]. The cyanate resin was extracted from

quartz-based prepregs and its curing behavior was

studied by dynamic DSC scans. Different isoconver-

sional approaches [48,49] were applied to analyze

the data and isothermal curing profiles were deter-

mined. The authors found the curing reaction to be

complex and to consist of four distinct stages, one of

which was of a zero-order reaction mechanism.

Processing of Cyanate EsterResins

Due to their unique property profile, cyanate

esters can be processed using a wide range of stan-

dard duroplastic manufacturing technologies includ-

ing processing technologies of fiber-reinforced

composites. For instance, among others, laminates

[24,30], prepreg manufacturing, resin transfer mold-

ing, filament winding, pultrusion [50], vacuum bag

processing, and sheet molding processing are appli-

cable. Foams [31] and foam sandwich composites

[51] have also been prepared from cyanate ester

resins. This is partly due to the favorable physical

state of the monomers or prepolymers employed

(typically solids of low melting points, sometimes

also liquids such as bisphenol E or bisphenol M

dicyanates). Prepolymers are typically obtained by

simply heating the monomers. Tacky semi-solids

are formed from monomers typically at conversions

of around 25�40%, whereas hard solids have con-

version degrees of 45 to 55% [52].

The flexibility with regard to processing technol-

ogies can further be exploited by using either blends

of cyanate ester resins with other high-performance

resins (typically epoxy and bismaleimide resins) or

by adding toughening polymers such as rubbers or

various types of thermoplastics.

Both monomers and prepolymers are usually well-

soluble in ketone-solvents such as acetone or methyl

ethyl ketone. For sulfur-containing monomers or

blends with (for example) BMI, more powerful sol-

vents may be required such as cyclopentanone and

mixtures of it with other solvents [52].

Storage stability of cyanate ester monomers and

prepolymers is also generally very good and can

easily reach 6 months (at room temperature) or

more (upon cooling). Low percentages (0.01 to 0.1

weight percent) of acidic stabilizers such as para-

toluene sulfonic acid or polyphosphoric esters may

be added to further increase the storage times.

However, impurities or blending with other materi-

als may significantly reduce the storage stability.

Curing under cyclo-trimerization can be brought

about with or without catalysts; however, thermally

induced curing without catalysts usually does not

yield satisfactorily high degrees of conversion.

Therefore, typically curing catalysts are used. The

most common systems are carboxylate salts and

chelate complexes of transition metals, which

mainly act by facilitating the cyclization reaction

by forming coordination compounds with the

monomers [37�39]. Many different metal cations

can be used. Since solubility of the metal catalysts

is usually not too good in the monomer mixtures,

the catalysts are usually applied in a co-solvent

based on an alkyl phenol such as, for instance, non-

ylphenol. This co-solvent also assists in the ring

formation reaction [52]. Besides transition metals,

urea compounds [53] and 2,20-diallyl bisphenol A[54] have also been used as catalysts.

The processing properties of the cyanate ester resin

system (pot life, gel time, total cure time) depend on

the catalyst composition, i.e. type and concentration

of the transition metal ion and application form (salt,

complex). Table 11.3 summarizes some effects of the

catalyst mixture on the processing properties. As a

rule of thumb, the preference for selecting a specific

metal ion has been suggested in the order

Cu21QCo21.Zn21.Mn21. Fe31.Al31, based

on the thermal stability properties of the achieved

cyanate ester resins (Table 11.3) [37,38,55].

However, the effect of the catalyst strongly depends

on the nature of the dicyanate ester system used and

choice of the appropriate systems needs to be care-

fully checked individually. More information on cata-

lyst selection is available in Hamerton, 1994 [37,38],

Lin et al., 2004 [6], and Li et al., 2006 [56].

The presence of residual metal catalyst is unde-

sired in the final resin since the metal ions decrease

both the hydrolytic [52] and thermal [57] stability

of the resin.

A typical casting procedure for cyanate resins is

as follows [37,38,52,53]:

“Zinc naphthenate (60�150 ppm metaI) isdissolved into the nonylphenol co-catalyst


(2 phr). The resulting catalyst solutionis added to the molten cyanate estermonomers or prepolymers at 90 to 110°C.The molten mixture is degassed undervacuum and cast into an aluminium platemold with Teflon gaskets and treated withsilicone mold release. The resins aregelled at 104 to 150°C and cured for 1 hat 177°C plus 1 h at 210°C plus 2 h at250°C [37,38].”

Properties of Cyanate Esters

The general property profile of cyanate ester

resins lies somewhere between that of BMI and

that of epoxy resins. This was nicely illustrated by

Hamerton in 1994 [37,38], who published the fol-

lowing, Figure 11.4. This figure displays the rela-

tionship of the glass transition temperature and

tensile elongation-at-break properties for these fam-

ilies of high-performance resins. It can be seen that

cyanate esters are well suited to fill in a technologi-

cal gap between the BMI and epoxy resin families.

Thermal, Mechanical, andElectrical Properties

Cyanate esters display good mechanical proper-

ties; for example, their tensile strength is typically

in the range of ca. 70 to 90 MPa, their tensile mod-

ulus is within 3.1 and 3.4 GPa, and their tensile

strain at break is between 2 and 5% [39]. Hence,

while tensile modulus is approximately in the order

of magnitude of typical epoxy and toughened bis-

maleimide resins, the values for both tensile

strength and especially for the tensile strain at

break are on average higher than the respective

values for typical epoxies and bismaleimide resins.

Furthermore, cyanate esters display unusually

high values for the glass transition temperature.

Additionally, compared to epoxy resins, cyanate

esters show improved behavior when subjected to

UL-94 burning tests (V-0 classification). The ther-

mal behavior of a variety of standard commercially

available cyanate ester resins as determined by

thermogravimetric analysis in the temperature range

from 300 to 900°C is reproduced in Figure 11.5

[41]. It is seen that all cyanate ester samples dis-

played a significant weight loss only at tempera-

tures significantly above 400°C; at average, the

cyanate esters had a peak mass loss rate tempera-

ture of ca. 468°C [41]. The highest char yields at

900°C of ca. 62% were obtained with novolac-

based cyanate esters whereas the lowest char yields

of around 30% were observed with the bisphenol M

and the dicyclopentadienyl cyanate ester resins.

The values of the dielectric constants for typical

cyanate ester resins lie in the range of 2.6 to 3.1,

which is comparatively low. Moreover, the loss

Table 11.3 Effect of Metal Catalyst on BADCy Resin Properties [37,38,52,55]

Metal Cu21 Co21 Zn21 Mn21 Fe31 Al31

Metal concentration (ppm) 360 160 175 435 65 250

Gel time (min)a

at 105°C 60 190 20 20 35 210

at 177°C 2 4 1 1 1.5 4

Conversion (%)b 96.6 95.7 95.8 93.8 96.5 96.8

Heat distortion temperature (°C)

dry 244 243 243 242 239 238

wetc 175 193 182 163 143 157

Moisture absorption (%)d 2.4 2.3 2.5 2.6 2.4 2.4

Thermal stability (hours)e 400 400 375 300 250 200

aNonyl phenol system used, nonyl phenol concentration not specified.bCure cycle 1 h/150°C12 h/250°C.cMoisture conditioned 64 h/92°C1 . 95% relative humidity.dAfter 500 h at 100°C.eIn 235°C/air, expressed as time required to reduce flexural strength by 50%.


100

90

80

70

60

50

40

30

300 400 500 600 700 800

Temperature (°C)

Mas

s fr

actio

n (%

)

900

PT-30XU-371BPCCE

F-10L-10M-10B-10XU-71787XU-366

Figure 11.5 Thermal behavior of various commercial cyanate ester resins as determined by thermogravimetric

analysis. The monomers used in the study were as follows: PT 30, phenol novolac cyanate ester [58]; XU-371,

phenol novolac cyanate ester [59]; BPCCE, bisphenol C cyanate ester; F-10, hexafluorobisphenol A cyanate

ester; L-10, bisphenol E cyanate ester; M-10, tetramethylbisphenol F cyanate ester; B-10, bisphenol A cyanate

ester; XU-71787, dicyclopentadienylbisphenyl cyanate ester; XU-366, bisphenol cyanate ester.

200

300

400

100

Tensile strain-at-break, %

Tetra-epoxide

Diepoxide

Dicyanate

Tg,

°C

0 21 3 4 5 6 7

ToughenedBMI

BMI

Figure 11.4 Relationship of the glass transition temperature and tensile elongation-at-break for BMI, epoxy,

and cyanate ester resins [37,38].


factors are also quite low, between 1 and 63 1023.

This renders them suitable for applications in the

electronics industry.

Table 11.4 summarizes some important physical

properties of commercially available cyanate ester

resins. These properties can further be modified, for

instance, by toughening the cyanate ester with elas-

tomeric additives.

Gelation of Cyanate Ester Resins

The gelation behavior of cyanate esters is also

influenced by the type of monomers used. Based on

Flory’s mean-field theory [58], the gelation point of

dicyanate ester/polycyanurate resins should occur at

a conversion of 0.5 [59]. However, in most cases

delayed gelation is observed depending on the

monomers used and in some cases gelation occurs

at conversions as high as 0.80 to 0.95 [59,60].

Typically, delayed gelation occurs when monomers

are used that have rigid backbone structures or

lower accessibility due to the steric demands of

bulky substituents. In contrast, flexible alkyl back-

bone structures or alkyl chains carrying electron-

withdrawing substituents often cause gelation more

closely to the expected theoretical value of 0.5.

Delayed gelation is also observed when the mono-

mer concentrations are rather low. Among other

causes, intramolecular cyclization of the difunc-

tional cyanate ester monomers has been suspected

to be a major reason for this behavior, and for the

case of bisphenol M dicyanate ester, it was experi-

mentally found that such monomer cyclization is

very likely to explain the observed behavior [59]. A

detailed literature survey covering the gel points of

a wide variety of monomers and a thorough discus-

sion on this subject is provided by Li and Simon

2007 [59].

Dimensional Stability UponCuring�Shrinkage Behavior

An important feature of cyanate ester resins

is their dimensional behavior upon cross-linking.

Typically, cyanate esters show an increase in spe-

cific volume at high conversion degrees. For

instance, when BADCy is cross-linked at 200°C the

forming polymer experiences a minor loss in spe-

cific volume only until a degree of conversion of

approximately 60% is reached; after solidification

at the gel point the solid mass slightly expands

again thereby counteracting potential inaccuracies

in mold reproduction and preventing shrinkage-

induced mechanical stresses in the network [39].

The relative changes in specific volume in depen-

dence on the conversion are reproduced in

Figure 11.6 [39]. Here, cyanate ester resins behave

very much unlike many other thermosetting sys-

tems like, for instance, unsaturated polyester resins

which show extraordinarily high shrinkage upon

cure especially after solidification and which there-

fore require the addition of significant amounts of

specially developed anti-shrink additives.

From the shape of the function of density versus

conversion depicted in Figure 11.6 it is immedi-

ately evident that suitable cyanate ester prepoly-

mers of low conversion degrees between 20 and

40% can be selected or prepared that show no over-

all volume change at all upon being cured to the

final degree of cross-linking.

Hydrolysis Behavior

Hydrolytic stability is an important prerequisite

for cyanate ester resins to be used as polymer matri-

ces in, for instance, printed circuit boards. When

stored under humid conditions and used without

prior drying, cyanate ester-based products may

delaminate or form blisters upon rapid heating to

elevated temperatures of around 250°C, which are

common when soldering such boards for electronic

applications. Although one of the advantages of

cyanate esters is their low water absorption (see

Table 11.5 for representative values of commercial

cyanate esters) compared to other high-performance

materials, they still may tend to blister more fre-

quently after shorter solder float times than do

epoxy resins [61]. The ultimate cause for this blis-

tering and delaminating is hydrolysis of the network

under formation of phenols, cyanuric acid, and car-

bon dioxide [61�63]. Kasehagen et al. [61] related

the blistering behavior of a cyanate ester resin to

dependence of the kinetics of the hydrolysis reac-

tion of the neat resin. For the cyanate ester resin

based on the 2,20-bis(4-cyanatophenyl)isopropyli-dene (DCBA) monomer cured with Mn21 octoate,

the authors found that up to temperatures, 220°Cno blistering occurred since the loss of water from

the laminate was sufficiently fast to prevent blister-

ing [61]. Although drying of the cyanate ester-based

materials before exposure to high peak temperatures

in the process is always an option to minimize


Table 11.4 Selected Physical Properties of Some Commercially Available Cyanate Ester Resins [39,41]

Name Linkage (X) Physical State,Viscosity

Properties of the Cured Polymers

Tga

(°C)GIC

b

(J/m2)Waterc

(%)Dk

d Dfe T5%f

(°C)Tmaxg

(°C)Charh

(%)

Bisphenol Adicyanate

CH3

CH3

Crystalline(MP5 79 °C)

257,289

140 2.5 2.91 0.005 443 468 39

Bisphenol Edicyanate

CH3

H

Supercooled liquid(MP5 29°C)100 cP at 25°C

258 190 2.4 2.98 0.005 455 479 47

Bisphenol Mdicyanate

CH3

CH3 CH3

CH3

Supercooled liquid(MP5 68°C)8000 cP at 25°C

192 210 0.7 2.64 0.001 439 482 31

Dicyclopentadienylbisphenolcyanate

c Semisolid1000 cP at 82°C

244, 265 70 1.4 2.80 0.003 447 463 33

Novolac cyanateester phenolictriazine CH2CH2

OCN

n

Semisolid20�40 cP at 80°C (for PT-15)250,000 cP at25°C (for PT-30)

300 to400

60 3.8 3.08 0.006 450 462 62

aGlass transition temperature [39,41].bHeat densitycWater absorption at saturation (100°C) [39].dDielectric constant [39].eDielectric loss (tanδ) [Kessler 2012] [39].f5% weight loss temperature [41].gPeak mass loss temperature [41].hChar yield at 900°C [41].

hydrolysis and hence delamination and blistering,

appropriate design of the polymer network may in

itself improve hydrolytic stability sufficiently well.

For instance, incorporating sterically demanding

substituents in the cyanate ester backbone (such as

methyl groups in para position to the cyanate group)

of the monomer has been shown to effectively

improve hydrolytic stability [61,64,65].

Properties of Cyanate EsterBlends with Epoxy and BMI

Cyanate ester resins are very often used in combi-

nation with numerous other high-performance poly-

mers [66,67,68,69], the most important being

duroplastic systems such as epoxy [3,4,21,69,70�78]

or bismaleimide resins [6�8,79�81,81a], or rubber

[82] of various thermoplastics [82] for improved

toughness.

Bismaleimide (BMI) resins are often modified by

cyanate ester resins to reduce the brittleness of the

BMI networks. To optimize the compatibility

between cyanate esters and other resins, special

monomers may be designed. For instance, monomers

that allow ready co-condensation with bismaleimide

resins have recently been prepared. The novel cya-

nate ester monomers carried allyl functionalities

in order to improve formation of interpenetrating

networks with BMI [83]. Other cyanate ester mono-

mers have especially been developed for tailored

composites such as, for instance, cyanate ester-

modified epoxy resin/glass fiber composites [4]. An

interesting strategy towards high-performance epoxy/

cyanate ester systems has been described recently

[5,25,71,72] where micro-capsulated composites of

cyanate and epoxy resins have been prepared and

shown to be of good mechanical and electrical per-

formance. Furthermore, cyanate ester systems con-

taining both epoxy and BMI resins such as

bismaleimide-modified bisphenol dicyanate epoxy

matrices have also been studied [84].

Epoxy resins are also known to catalyze cyanate

ester cure. This latter effect has been ascribed to

1.00

0.99

0.98

0.97

0.96

0.95

0.94

0.93

0.920 20 40 60 80 100

% Conversion

Gel

Measured at 25°C

Spe

cific

vol

ume

Measured at 20°C

Figure 11.6 Relative changes in specific volume of cyanate ester resin based on BADCy monomer cured at

200°C.


the hydroxyl groups present as impurities in virtu-

ally all epoxies [37,38]. The preparation of blends

from cyanate ester resins with epoxy resin can be

performed quite easily by blending the two resins

at elevated temperatures between 80 and 100°C,adding the catalyst formulation, degassing the mix-

ture by applying vacuum, pouring the resin blend

into a mold, and curing the system by, for instance,

heating to about 180°C. As in the above case of

neat cyanate resin cure, a post-curing phase at even

higher temperatures above 200°C can follow. With

increasing amounts of cyanate ester resin in the

blend the more likely a post-curing phase needs to

be included in order to bring about complete resin

conversion [37,38].

For processing cyanate ester resins and

cyanate ester resin blends, the following compo-

nents may be present in the cyanate ester resin

formulations [85]:

• Cyanate monomer and its prepolymers.

• Epoxy prepolymers such as bisphenol A digly-

cidyl ether (BADGE); N,N,N0,N0-tetraglycidyldiamino-4-40-diphenylmethane (TGDDM); N,

N,O-triglycidylamino-4-phenol (TGAP).

• Bismaleimides (BMI, BT resins).

• Catalyst.

• Thermoplastics.

• Core-shell rubbers.

• Reactive rubbers.

• Reactive diluents.

• Fibers: glass, carbon, aramid, or other.

• Other fillers or reinforcements (see Introduction).

Since it is impossible to summarize and appro-

priately acknowledge all major developments that

have taken place during the past decades, some

typical properties of selected examples of classic

BMI and epoxy-based blends shall be given in

Table 11.5 to illustrate the range of application

properties possible.

The synthesis and characterization of different

novel cyanate/epoxy and cyanate/BMI blends have

also been described more recently. For example,

the results reported by Sarodajevi et al. [1] illustrate

very well the great potential that cyanate esters do

have with respect to modifying the property profile

of epoxy and BMI resins. The authors synthesized

Table 11.5 Properties of Three Cured Cyanate Ester Resin Blends Based on BADCy Monomer and EitherEpoxy (BADGE, DGETBBPA) or BMI [52]

BADGEa (57%) DGETBBPAb (25%) BMIc (10%)

Cyanate ester (weight %) 43 75 90

Catalyst type Cu acetylacetonate Cu naphthenate Zinc octoate

Catalyst amount 0.05 0.24 0.02

Nonylphenol 2 2 5

Maximum cure temperature (°C) 177 215 232

Cured resin properties

Heat distortion temperature (°C)

Dry 192 221 248

Wetd 178 181 170

Water absorptiond(%) 1.31 1.65 2.02

Flexural strength (MPa) 117 173 131

Flexural modulus (GPa) 3.1 3.3 3.3

UL 94 flammability Fails V-0 Fails

aBADGE: bisphenol A diglycidyl ether.bDGETBBPA: diglycidyl ether of tetrabromobisphenol A.cBMI: bismaleimide.dMoisture conditioned 64 hours at 93°C plus.95% relative humidity.


a series of novel cyanate ester resins, performed

blending experiments with epoxy and BMI resins,

and characterized the resulting materials with

regard to the mechanical and thermal properties [1].

With all blends based on new monomers studied

by the authors, the performance of the cyanate-

modified epoxy resin systems was generally signifi-

cantly better than that of the neat epoxy or BMI

materials. For instance, in the case of the epoxy

systems, the tensile strength of the cyanate ester

blends varied between 55 to 76 MPa (compared to

52 with the neat epoxy); the flexural strength varied

between 96 and 169 GPa (neat epoxy: 89 GPa); and

the impact strength varied between 1.3 and 4.5 J/m

(neat epoxy: 0.67 J/m). With respect to the thermal

properties, the temperature stability was improved

in most cases by up to 30°C higher temperatures

as expressed by the T10 value (358°C for neat

epoxy, 390°C for BCAM-modified cyanate ester/

epoxy product). Also the char yield typically

increased [1].

Similarly, the mechanical and thermo-

mechanical performance of BMI was greatly

improved upon blending with a newly prepared

cyanate ester. The effect depended strongly on the

amount of cyanate ester used. By addition of 9 parts

cyanate ester resin to 91 parts of BMI, the tensile

strength, for instance, was enhanced from 418 MPa

to 461 MPa. At lower levels, performance improve-

ment was accordingly less pronounced. A similar

trend was found with other mechanical characteris-

tics like tensile modulus, flexural strength, flexural

modulus, fracture toughness, and impact strength.

The findings are summarized in Table 11.6. As evi-

dent from the data given in the table, only minor

amounts (,10%) of the cyanate ester are needed to

significantly influence, i.e. improve, the overall

performance of the BMI. The chemical structure of

the novel monomer used in this work for the BMI

blending experiments is given in Figure 11.7.

Recent Developments

Novel Cyanate Ester Monomers

Besides those materials that are already used on

a large scale, a huge number of special monomers

have been prepared during the past decades and

are described in the scientific literature to further

enhance specific property profiles. Although they

are mostly not yet employed commercially, a few

recent examples are briefly addressed in this sec-

tion to illustrate some rationales behind the design

of such monomers with respect to their modifying

effect on the material properties. Often, blends and

composites are produced from the novel cyanate

esters and only the performance of the overall sys-

tem is given in the scientific literature. Hence,

unfortunately, it is difficult to compare the perfor-

mance of all monomers and resin systems described

in the literature within one single table. The follow-

ing examples feature new monomers that have been

shown to influence network rigidity and in turn

toughness and thermo-mechanical behavior of the

polymer, and the flammability of cyanate ester

resins. In addition, following the increasing trend

towards sustainable manufacturing technologies

in the chemical industry (“green chemistry,” “bio-

based materials,” etc.), a subsection has been added

that features some recent developments with regard

to the use of renewable resources to prepare novel

bio-based cyanate resins.

Monomers Modifying Flexibility

Anuradha and Sarodajevi prepared a series of

structurally closely related novel cyanate ester mono-

mers containing propylene chain segments between

the aromatic cores of the diphenylcyanate units in

order to improve the flexibility of the obtained cyanic

ester resin networks [83]. The chemical structure of

the newly prepared monomers is depicted in

Figure 11.8. The idea was that the flexible aliphatic

methylene spacer should lead to a moderate improve-

ment in toughness of the resulting thermoset accom-

panied by a decrease in dielectric properties [83].

It has been shown that increasingly bulky or longer

hydrocarbon bridges between the aromatic rings

lower the dielectric properties; the dielectric constant

progressively decreased when the alkyl bridges were

changed from methylene to isopropylene to dicyclo-

pentadiene to meta-diisopropenylbenzene. The reason

for this is that alkyl substituents of increasingly

demanding steric requirements reduce the spatial den-

sity of highly polar groups responsible for the dielec-

tric interactions present in the volume unit of the

polymer network and, additionally, decrease the

electronegativity of the benzene rings by their

electron-donating capacity [37,38]. It was found fur-

thermore in earlier studies that especially substitution

by methyl groups in the ortho position to the cyanate


Table 11.6 Improvement of Properties of BMI by Modification with Different Amounts of Cyanate Ester Resin [1]

BT Ratio Mechanical Properties Dynamic Mechanical Properties

Cyanatea:

BMI

Tensile

Strength

(MPa)

Tensile

Modulus

(GPa)

Flexural

Strength

(MPa)

Flexural

Modulus

(GPa)

Fracture

Toughness

(J/m2)

Impact

Strength

(J/m)

Glass

Transition

(°C)

tan δ Storage

Modulus

drop

Temperature

(°C)

Full Width

at Half

Maximum

(°C)

100:0 516 48 622 69 212 1412 280 0.140 212

(12,560 MPa)

50

9:91 461 35 554 59 136 1218 333 0.126 304

(22,870 MPa)

62

6:94 442 33 545 58 118 1175 369 0.121 320

(23,450 MPa)

57

3:97 422 32 524 56 104 1121 381 0.112 345

(24,750 MPa)

54

0:100 418 29 516 54 79 1039 . 400 0.063 383

(25,600 MPa)

n.a.

aCyanate resin based on bis(40-cyanato-30-methoxybenzal)-4,40-diaminodiphenyl methane (BCDM) as the monomer.

group significantly lowered the overall dielectric con-

stant by shielding and weakening the CaO, CaN,

and CQN dipoles [37,38,83].

With the new monomers Cy(a), Cy(b), Cy(c), Cy

(d), and Cy(e), it was found that both the glass tran-

sition temperature and the thermal stability fol-

lowed the trend Cy(b).Cy(c).Cy(a).Cy(e).Cy(d). The authors explained their findings by the

symmetry of the used structures, the TG being high-

er for those structures with the para-substituted

rings. Higher symmetry is also directly correlated

to a higher cross-linking density, and in turn, to

better thermal stability. Although all prepared cya-

nate esters displayed LOI values in the range from

36.1 to 38.7 and hence were considered to be suffi-

ciently thermally stable, the cyanate esters derived

from the monomers Cy(b) and Cy(c) having the

highest degree of symmetry were thermally the

most stable within the series [83].

Possibly the higher symmetry of the cyanate

ester monomer packing units also led to the highest

glass transition temperature observed with the

para-tetraaryl cyanate ester monomer within the

series of structurally related monomers investigated

by Reams and Boyles [86] and depicted in

Figure 11.9. Again, the more symmetrical tetraaryl

monomers yielded significantly higher glass transi-

tion temperatures, storage moduli, and cross-linking

densities than the corresponding triaryl monomers

of lower symmetry. Within each series, the para-

substituted monomers showed larger values than

the meta-substituted, and in turn, higher values than

the ortho-substituted monomers [86].

Guenthner et al. recently prepared a cyanate ester

resin of greatly improved flexibility that was based

upon the novel monomer 1,2,3-tris(4-cyanatophenyl)

propane [87]. The approach used by the authors illus-

trates nicely a general strategy to improve the flexi-

bility in rigid aromatic three-dimensional polymer

networks by incorporation of a monomer of

improved molecular mobility. The three-dimensional

polymer network obtained with the new monomer

resulted, according to the authors, in a “checker-

board”-like network architecture in which flexible

juncture points alternated with rigid juncture points.

H3C

H3C

H3C

H3C

CH3CH3

CH3

CH3

OCN

NCO

NCO

Cy(c)

Cy(a)

Cy(b)Cy(e)

Cy(d)

NCO

OCN

OCN

OCN

OCN

OCN

OCN

CH3

H3C

O

O

OO

O

Figure 11.8 Series of structurally related cyanate ester monomers used by Anuradha and Sarodajevi [83].

NCO

H3C

CH3

O

O

OCNN

N

Figure 11.7 Chemical structure of bis(40-cyanato-30-methoxybenzal)-4,40-diaminodiphenyl methane

(BCDM) [1].


Figure 11.10 shows an ideal representation of the

resulting molecular framework in comparison to the

network structure obtained from polymerization of

the commercially available, novolac-based cyanate

ester monomer Primaset® PT-30 [87]. The new

material had the same cross-linking density com-

pared to the Primaset® PT-30 polymer, but displayed

several advantages over the commercial material:

under identical curing conditions, the activation

energy for autocatalytic thermal cure at 210�290°Cwas lower by 14 kJ/mol21, the overall extent of cure

was higher with the new material while cross-link

density at full cure (3.8111 /2 0,0043 1021) and

wet glass transition temperature (ca. 245°C) was

approximately the same. The novel material had

higher molar volume and lower density.

More details on the influence of chemical struc-

ture of monomers on the resin performance are

available in Hamerton and Nair et al. [37,38,40].

The structure-reactivity and structure-property rela-

tionships between a wide range of different cyanate

ester monomers studied until 1994 has been exten-

sively discussed in Hamerton [37,38]; the literature

until 2001 has been covered well by Nair et al.

[40]. Since then, most work on cyanate esters has

focused on formulation and development of cya-

nate ester blends with other duroplastic polymers

and on the characterization of a wide range of cya-

nate ester composite materials including nanocom-

posites and comparatively less effort has been

invested in the characterization and design of new

monomers.

Monomers Conferring Flame Retardance

The flame retardancy of cyanate esters is

frequently being improved by incorporation of

bromine-substituents into the polymeric network

or by formulation of cyanate ester resins with

bromine-based additives. Due to the inherent tem-

perature stability of cyanate esters, generally, lower

bromine levels are required as for other polymers.

Compared to, for instance, covalently bromine-

modified epoxy resins which require about 20% of

bromine to yield a flame retardant material, only

ca. 12% of bromine are required with cyanate

esters, and in unmodified cyanate resin formula-

tions lower levels of brominated additives are

usually required as well to achieve a UL94 V-0

rating [88].

CH3 CH3

CH3

CH3 CH3

CH3H3C

H3C

H3C

NCO

NCO

NCO

NCO

NCO

OCN

OCN

OCN OCN

OCN

OCN

OCN

H3C

H3C

H3C

Figure 11.9 Series of structurally related cyanate ester monomers used by Reams and Boyles [86].


NCO

N

N

N

N

N N

N

NN

N

N

N N

NN

N

NN

N

N N

N N

N N

O

O

O

O

O

O

O

O

O O

O

O

O

O

O O

O

O O

O

O

O

O

O

O OO

OO

O O

O

O O

O

O

O

O

O

O

O

O

O

O

O

OO

N N

N N

N

N

N

N

N

N

N

N

NN

N

N N

OCN

OCN

OCN

OCNNCO

(a)

(b)

O

Figure 11.10 Structural comparison of idealized network architectures obtained with Primaset® PT-30

monomer and the novel monomer 1,2,3-tris(4-cyanatophenyl)propane [87].


However, incorporation of bromine into cyanate

esters not only improves polymer performance but

has some drawbacks as well, such as the potential

corrosion of metal components in composites due

to halide ion liberation, a decrease in glass transi-

tion temperature leading to a softening at already

lower temperatures, a possible reduction of the

thermo-oxidative stability, a deterioration of the

dielectric loss properties, and, due to the high

atomic weight of bromine, an increase in the spe-

cific weight of the polycyanurates [88].

As an alternative, it has been shown that incor-

poration of phosphorous can also greatly enhance

the flame retardant properties of cyanate ester

resins and many phosphorous-containing com-

pounds have been prepared [6,89,89a,90].

Other novel cyanate ester monomers with promis-

ing flame retardant properties that carry phosphorous

moieties have been synthesized more recently.

For example, Hamerton et al. [88] in a four-step syn-

thesis prepared 2,2,4-tri(4-cyanatophenoxy)-4,6,6-tris-

phenoxy-2λ5,4λ5,6λ5-[1,3,5,2,4,6]-triazatriphosphine(compound x, Figure 11.11), a new monomer con-

taining a six-membered ring consisting of phospho-

rous and nitrogen atoms that confers improved flame

retardancy to the cured cyanate ester network and

simultaneously avoids the aforementioned drawbacks

caused by bromine substituents. The new synthetic

route yielded the monomer with a yield of ca. 35%.

A one pot-synthesis leading to phosphinated

bisphenols by addition of 9,10-dihydro-9-oxa-10-

phosphaphenanthrene-10-oxide (DOPO) to either

1,1,1-tris(4-hydroxyphenyl)ethane or diaminophenyl-

methane- and diaminophenylether-based benzoxa-

zines with subsequent conversion of the bisphenols

to the corresponding dicyanate derivatives has

been described [91,92]. The structures of the

obtained cyanate ester monomers are depicted in

Figure 11.12. The latter two compounds can be pre-

pared without any catalyst at room temperature in

high purity without particular purification that holds

some promises for the procedure for larger scale

applications. The resulting polycyanurates showed

good flame retardancy as well as good dielectric

properties.

Zhang et al. [12] prepared a novel substituted

biphenyl-based cyanate ester resin (Figure 11.13).

The monomer, 4,40-bis(4-cyanatobenzyl)biphenyl(BBPCy) does not contain any phosphorous or hal-

ogen atoms and hence, avoids the drawbacks

involved with such compounds like, for instance,

the generation of toxic fumes upon thermal

combustion of halogen-containing compounds.

Compared to a resin derived from the commercially

available monomer BADCy, with the new mono-

mer a UL-94 rating of V-0 was reached and both

glass transition and thermal stability were slightly

better (Tg5 258°C). The low water absorption

(1.08 %) and excellent dielectric properties (Dk:

2,94, Df: 0,0037 at 1 GHz) achieved with this mate-

rial provide promising properties for applications in

electronic encapsulation and printed circuit board

manufacturing [12].

Bio-Based Raw Materials

Currently much research work is performed with

the general direction of modifying the raw material

basis of the chemical industry with regard to

re-generable resources. Raw material shortages and

raw material costs are an important incentive to

exploit natural resources (plants and plant oils) as

the basis for new materials in many fields. This

trend is also affecting the research work in the cya-

nate ester resin field where monomers based on nat-

ural resources are increasingly being investigated.

Recently, strategies to prepare cyanate esters

from fully or partly re-generable raw materials

have been proposed [93�95]. For example, one

approach recently described uses epoxidized soy-

bean oil (ESO) as a bio-based modifier for com-

mercial cyanate ester monomers [95]. The authors

reacted 1,10-bis(4-cyanatophenyl)ethane with ESO

together with a silicon-organic compound that led

to in situ generation of nanosilica in a sol-gel pro-

cess. The nanocomposites thereby accessible are

NCO

O O

O

OO

P

P

P

N

N

NO

NCO

OCN

Figure 11.11 Chemical structure of 2,2,4-tri

(4-cyanatophenoxy)-4,6,6-tris-phenoxy-2λ5, 4λ5,6λ5-[1,3,5,2,4,6]-triazatriphosphine [88].


partly based on renewable resources and showed

improved storage moduli and thermal stability com-

pared to neat cyanate ester reference material.

A different approach was described by

Meylemans et al. [93]. The authors ultimately used

lignin as a polymeric raw material basis that is

accessible from the wood-based industry; it repre-

sents a natural polyphenolic resin of varying consti-

tution and composition. Lignin has since long been

tested as a natural-based substitute for phenolic

monomers or oligomers in phenolic resins [96,97]

and numerous other polymers such as polyesters

[98,99], polyurethanes [100], and epoxy resins

[101�103]. Due to its high carbon-content, it has

also been investigated as a precursor for carbon

fibers [104]. Hence, it seems reasonable to inves-

tigate this versatile raw material as a source

for monomers applicable in cyanate ester resin

production.

The authors suggest producing well-defined

phenolics such as vanillin and, in turn, creosol,

from conversion of lignin (Figure 11.14a), and,

by dimerization, further transforming these into

bisphenols, which are then suitable as cyanate

precursors for resin production (Figure 11.14b)

[93,105].

Davis et al. recently demonstrated that trans-

anethole can be chemically transformed into

suitable precursor molecules for the preparation

of cyanate ester resins that show a technological

performance comparable to currently available

commercial resins [94]. This approach is highly

interesting since it exploits re-generable plant oil as

an alternative resource for phenolic compounds.

Trans-anethole is the major ingredient (to about

90%) of the essential oil of star anise (Illicium

verum) and can be obtained from the crude oil.

OCN

O

N NCO

P O

O O O

O

O

O O

OCN

P

P

P

NCO

NCO

H3C

N NOCN

O

(a)

(b)

(c)

OP

N

Figure 11.12 Chemical structures of the DOPO modified cyanester monomers used by Chang et al. [92]

(structures a and b) and Lin et al. [91] (structure c).

OCN

NCO

Figure 11.13 Chemical structure of 4,40-bis(4-cyanatobenzyl) biphenyl (BBPCy) [12].


OH

(a)

OO

OHO

N

N

N

N

N

N

NN

N

NN

N N

HO

O

O

O

O

O

OR

OH

OH

OH

CNBr/NEr3-20 °CCNBr/NEt3-20 °C

Zm(Ac)2/CH2O H+/HCOR

OH OH

R

R

OH

OH

OCH3

OCH3

OCH3OCH3

OCH3 OCH3

H3CO

H3COH3CO

H2/cat.

Creosol

Creosol

Lignin model

Vanillin

H3CO H3CO

H3CO H3CO

H3CO

H3CO H3CO

H3CO

1OCN OCN OCN OCN

OCH3

OCH3

Polycyanurate thermosets

OH

OH

(b)

R = H(2), Me(3), Et(4)

Δ

Δ

Figure 11.14 Conversion of lignin into re-generable precursor molecules (a) that are suitable for producing

cyanate monomers (b).


The oil is produced in amounts of approximately

400 metric tons per annum for the preparation of

flavor and fragrance products, which of course is

not much compared to the petrochemical-based

manufacturing of phenols or even to the amounts of

lignin that are amounting as byproducts from the

wood industry; however, exploiting new high-

performance applications of high added value might

possibly act as an incentive to increase the crop

production and hence improve availability in the

long term [94].

The chemical structure of trans-anethol and the

cyanate ester monomers obtained by chemical trans-

formation are given schematically in Figure 11.15.

Especially the properties of the cyanate ester

resin obtained from RRR/SSS 1-ethyl-2-methyl-3-(4-

cyanatophenyl)-5-cyanatoindane displayed a remark-

ably high glass transition temperature of 313°C,which was superior to the LECy-based commercial

reference cyanate ester; further, it had a lower water

uptake (1.66 %) than for the two LECy and BADCy

reference cyanate esters. The glass transition temper-

ature measured after 96 h immersion in water at

85°C was still 223°C, which was somewhat lower

than for the commercial reference materials, but still

within an acceptable range. The cyanate ester resin

obtained from the other monomer, racemic 1,3-bis(4-

cyanatophenyl)-2-methylpentane, displayed compar-

atively inferior properties [94].

Recently Covered Aspects ofProcessing Cyanate Ester Resins

In the following subsections, a selection of a

few recent developments in alternative processing

methods of cyanate esters are briefly discussed,

including the preparation of cyanate ester resins

via frontal polymerization, microwave-assisted

polymerization, plasma polymerization of cyanate

monomers, and the preparation of cyanate esters

under steric restrictions with nanoconfinement in

the presence of controlled porous glass.

Frontal Polymerization

The curing of cyanate esters is accompanied by

the development of high exothermic enthalpies.

The reaction heat generated is usually dissipated by

appropriate tooling design to avoid inhomogeneous

curing or defect formation. However, the curing

H3C

H3CO

Trans-Anethole

CH3

CH3

CH3

CH3

H3C

H3C

OCNNCO

Racemic 1,3-Bis(4-cyanatophenyl)-2-methylpentane

NCO

RRR/SSS 1-ethyl-2methyl-3-(4-cyanatopheny)-5-cyanatolndane

NCO

OCN

OCN

Figure 11.15 Chemical structures of the precursor molecule trans-anethole from the natural plant oil of Illicium

verum and cyanate ester monomers derived from it [94].


exothermy may also be used with advantage for

inducing and advancing the thermal cure in a

cyanate ester reaction system. Frontal polymeriza-

tion is a technique that makes use of the reaction

exothermy generated during polymerization. Hence,

recently, frontal polymerization technology has

been described and studied for the preparation of

cyanate esters [106,107].

Frontal polymerization means that within a solid

or highly viscous polymerization system the reac-

tion heat generated within a distinct reaction zone

where polymerization has been initiated by, for

instance, an external heat source, serves as the ther-

mal stimulus to initiate and propagate the polymeri-

zation further throughout the rest of the system.

The external heat source starting the overall reac-

tion sequence may be either some sufficiently hot

object such as, for instance, a soldering iron that is

being contacted with the outer zone of the solid

reaction system, or it may be another chemical

reaction system in close proximity that is able to

deliver sufficient exothermal reaction enthalpy for

initiating the polymerization reaction in question.

Figure 11.16a schematically shows an example

for the experimental set up of a typical frontal poly-

merization system. Figure 11.16b illustrates the

propagation of the polymerization front of ther-

mally induced formation of Primaset LeCy cyanu-

rate ester polymer [106].

In Figure 11.16a, the two highly viscous reaction

mixtures are brought into close contact and an

external heat source (which is on a laboratory scale,

for instance, a heated soldering iron) is contacted at

the juncture where the two phases meet. Typical

contact times may be a couple of seconds [106].

This initial introduction of heat is sufficient to start

the polymerization sequence of the initiator system,

which delivers sufficient exothermal energy to

initiate the polymerization of the cyanate ester

polymerization. Acrylate systems may generate

interface temperatures around 250°C [108]. Since

for cyanate ester resin formation initial reaction

temperatures. 200°C are required [109,110], acry-

late polymerization may be a suitable initiator sys-

tem. As the front of the initiator chemical reaction

proceeds horizontally, the cyanate ester polymeriza-

tion is progressively initiated and develops a verti-

cal propagation zone.

Figure 11.16b shows an example where no

chemical initiator system is used but the reaction

front of cyanate ester polymerization is started by

short contact with a heated surface of a

temperature. 500°C. The progression zone is

clearly visible from the photograph. The author has

found that chemical initiation is of advantage in

cases where too much heat is introduced into the

reaction system. In such cases the excess energy

introduced leads to solvent or monomer evapora-

tion, which causes bubbles that in turn act as heat

sinks and hamper homogenous propagation of the

reaction front [106].

The potential advantages of frontal polymeriza-

tion techniques are that rapid curing of polymers

can be achieved without the need for external

heat sources. Frontal polymerization may provide

homogenous curing of thick samples of highly vis-

cous systems. It may be applied as a means of cur-

ing solvent-free systems that may be applied to

fill or seal structures with cavities of arbitrary

shapes that are otherwise difficult to appropriately

seal off. By utilizing the reaction enthalpy as a self-

sustaining energy source, the need for external heat

sources is circumvented.

Microwave-Assisted Polymerization ofCyanate Esters

Conventionally, the energy required to initiate

polymerization is introduced thermally. However, an

interesting option may also be to use microwave

radiation to bring about polymerization. Since the

electromagnetic radiation completely penetrates the

bulk material, the energy is introduced more

uniformly and the batch is heated throughout the

bulk more homogeneously than with convection.

The thermal conductivity of the material is also not

of importance. Since heating of the whole bulk is

brought about instantaneously upon switching on the

microwave generator, the curing reaction should be

accelerated as compared to conventional heating.

Microwave-assisted curing hence may lead to more

quickly and uniformly cured cyanate ester products.

However, it was found that microwave radiation was

not feasible for the processing of unmodified cyanate

ester resins [111]. It was found that either charred

or incompletely cured products were obtained.

Obviously, by microwave radiation the reaction was

accelerated so much that the liberated exothermal

energy caused complete thermal degradation of the

formed material. On the other hand, when the reac-

tion conditions were more moderately selected to

contain microwave-assisted auto-acceleration, only


incompletely cured products were the result.

However, when the authors used thermoplastic-

modified networks by co-reacting phenolphthalein-

based poly(arylene ether) as a reactive thermoplastic

toughness modifier, the reaction was much more

controllable and led to resin materials of very high

degrees of conversion (97�98%) within minutes of

reaction time. Furthermore, it was found that by

applying microwave heating, defined morphologies

of the thermoplastic-modified cyanate ester resins

could be obtained independent of the applied micro-

wave input power. In general, the faster the curing

took place, the finer was the resulting network mor-

phology [111]. Hence, microwave-assisted proces-

sing of cyanate ester resin systems may in some

cases be employed beneficially in order to further

reduce the processing times and create products of

defined morphology.

(b)

Unreacted acrylatemonomer

Front propagation of acrylate polymerization

Reactionzone 1

Unreacted cyanate ester monomer-initiator mixture

Reaction zone 2

Hot cyanate ester polymer

Hot acrylate polymer

Fron

t pro

paga

tion

of c

yana

tees

ter

poly

mer

izat

ion

Site ofcontact withexternal heat source

(a)

Figure 11.16 Schematic representation (a) of the frontal polymerization principle using an exothermal

chemical reaction (e.g. in this case an acrylate polymerization) for the thermal initiation of cyanate ester resin

synthesis [106], and (b) specific example for the reaction sequence during frontal polymerization of a thermally

induced cyanate ester reaction [106].


Nanoconfinement

Cyanate esters have been synthesized using

nanoporous templates based on controlled pore

glass of different pore sizes [112�114] to modify

the properties of the resulting polymers.

Nanoconfinement often leads to distinct differences

in the reaction rates at which a synthesis takes

place and due to the imposed steric restrictions

within the nanoreactor systems often results in

reaction pathways that cannot be realized via tradi-

tional bulk synthesis. For instance, when oxirane-

modified fullerene molecules (C60) are intercalated

and trapped within single-wall carbonanotubes that

then act as nanoreactors, unusual reactions are pos-

sible such as the formation of linear, unbranched

polymeric chains of C60O-molecules. Without

nanoconfinement, three-dimensional products are

obtained [115].

When cyanate ester synthesis is performed under

such restricted steric conditions, the reaction rate

of cyclotrimerization was found to be greatly

enhanced by a factor of ca. 20 when hydrophobic

pores of 11.5 nm diameter were used as confine-

ment with bisphenol M dicyanate as the monomer

[113,114]. The authors found that no alteration of

the reaction mechanism took place and they also

excluded that intramolecular cyclization of the

difunctional monomers was mainly responsible for

the observed effects [112]. The glass transition tem-

peratures of the obtained cyanate ester products

were lower as compared to the bulk polymerized

compounds, and the TG depression increased with

increasing degree of conversion. Since it was great-

est with the fully reacted product (i.e. the resin

where the maximum achievable conversion was

obtained), the authors concluded that the molecular

stiffness influenced the magnitude of the nanocon-

finement effects.

Synthetic routes to cyanate esters using nanocon-

finement may be of future use to prepare novel and

yet unexpected thermal properties by appropriate

modification of the reaction conditions. Thus, not

only modification and substitution of cyanate

ester monomers and co-monomers, but also novel

preparation techniques may yield interesting new

properties.

An interesting example for producing a nanos-

tructured material with high potential for the use in

biosensing from cyanate esters via nanoconfine-

ment of the cyanate ester monomers within a

template has recently been described by Gitsas

et al. [116]. The researchers produced a nanorod

array from cyanate ester resins by using self-

ordered anodic aluminum containing arrays of

aligned nanopores of defined pore diameter.

The monomer formulation was loaded into

the nanoporous template and cured at high tem-

perature. The polycyanurate nanorod array was

used for optical waveguide spectroscopy of taurine

binding to the surface groups of the nanorod

assembly [116].

Plasma Polymerization

Cyanate ester resins may also be generated as

thin surface films on various substrates using

plasma polymerization techniques. For instance,

thin films of polymeric polynitriles based on

4-cumylphenol cyanate ester (PPCPCE) were

deposited onto silicon wafers and quartz glass

[117,118]. The smooth and homogenous PPCPE

films showed low dielectric constant (εr5 2,51) and

have some potential as novel intermetallic dielec-

trics in microelectronics. The dielectric constant of

the thin film was lower when lower (5 W) rather

than higher (10, 20 W) discharge power was used

for the deposition; this was attributed by the authors

to the higher retention of aromatic structure and

lower concentration of radicals in the deposited film

at the lower discharge power [118]. Furthermore, at

higher discharge powers, the observed films had a

significantly rougher surface [117].

Applications

Cyanate esters are mainly employed in electron-

ics, aerospace, space, transportation, and industrial

applications as matrices for composites, coatings,

adhesives, and encapsulating resins. Although they

have a favorable combination of properties, so far,

the use of cyanate esters, due to their comparatively

high cost, is restricted mainly to high-performance

applications in sectors where cost is a secondary con-

cern. A typical product area for cyanate esters,

hence, is as structural parts in aerospace applications.

Many primary and secondary structures in military

aircraft are produced from cyanate ester-based com-

posites; for example, the Dassault Rafale fighter

jet incorporates high-temperature stable cyanate

ester resin in its composite parts (Figure 11.17a).


Other examples for applications of cyanate esters in

airplanes are air ducts or critical electrical compo-

nents close to the engine where they are exposed to

temperatures as high as 235°C. Cyanate esters are

also found in high-speed vehicles such as Formula

One racing cars or in motorbikes as, for instance,

muffler systems [120].

Radomes are also frequently based on cyanate

ester resins due to their favorable dielectric behav-

ior. Other aerospace applications include structural

parts for satellites (Figure 11.18a), antennas, reflec-

tors, optical elements, high-precision detectors,

solar array substrates, and other components. For

instance, NASA has based space carriers that are

used to transport cargo into space on cyanate ester-

based composite materials. As an example, the

Super Lightweight Interchangeable Carrier (SLIC)

shown in Figure 11.18b relied largely on carbon

fiber-reinforced cyanate ester resin. By employing

such reinforced cyanate ester materials, lower

launch costs (due to the lower weight) and better

dimension stability can be achieved as compared

with metals. Compared to epoxy-based systems,

cyanate esters display lower tendency towards

micro-crack formation, especially when they are

toughened by addition of minor amounts of an

appropriate toughening agent. Furthermore, cyanate

esters show much greater stability towards ionizing

radiation in comparison to other high-performance

polymers; they can withstand irradiation dosages as

high as 109 rads [120].

Interesting cyanate ester [122] and cyanate ester/

epoxy blends [46] have recently been described

that display shape memory behavior. In the latter

case, to obtain the shape memory characteristics,

poly(tetramethylene oxide) was functionalized with

phenol and incorporated into the three-dimensional

epoxy/cyanate network. Such shape memory high-

performance polymers are very promising for space

applications as components made therefrom can be

compacted during the launch phase and unfolded in

space.

However, perhaps at present the most commer-

cially important field of applications include the

manufacturing of electronic and low-dielectric

semiconductor devices. An example for a cyanate

ester-based electronics enclosure is given in

Figure 11.19a. Approximately 70% of the manufac-

tured high-speed printed circuit boards are already

based on cyanate ester resins [40]. For applications

in the microelectronics industry, the low dielectric

constant and low dissipation factor of cyanate ester

resins plays an important role for their widespread

application. These special dielectric properties

allow for better electrical performance, higher sig-

nal speed, and lower power loss as compared with

other materials [40]. Hence, the single largest use

for CEs is the lamination of substrates for printed

circuits and their assembly via prepreg adhesives

into high-density, high-speed multilayer boards

(Figure 11.19b) [37,38,40].

Future applications of cyanate esters are seen as

aircraft interiors (Figure 11.17b) such as flooring,

ceiling, and walls with tight smoke and heat regula-

tions, as high-performance powder coatings, auto-

motive electronics (navigation) systems, as next

generation materials for high voltage industry (e.g.

generators, etc.), as shielding materials in nuclear

plants (due to their extraordinary radiation resis-

tance), as high-end friction materials, and as adhe-

sives especially for lightning strike applications or

in high-temperature rapid bonding [120]. Cyanate

esters will also find applications as wave guides

and in non-linear optics.

(a) (b)

Figure 11.17 Use of cyanate ester resin-based materials in aerospace applications [119,120].


(a)

(b)

Figure 11.18 Use of cyanate ester-based materials in space applications [120,121].


Trade Names

Only a few companies are currently capable of

producing cyanate ester resins on a commercial

scale; one reason for this may be the extremely

hazardous nature of the starting materials (cyano-

gen bromide or cyanogen chloride) used to obtain

the monomeric compounds and the correspondingly

demanding production technology [39]. Table 11.7

lists the most common trade names for commercial

cyanate ester resins. Only the economically most

important suppliers are considered.

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Table 11.7 Trade Names and Basic Structural Elements of Some Commercial Cyanate Ester Resins [16,39]

Trade Name Structural Characteristics Supplier Reference

Primasett BADCy Bisphenol A dicyanate ester Lonza Kessler (2012) [39]

Primasett LECy Bisphenol E dicyanate ester Lonza Kessler (2012) [39]

AroCyL-10 Bisphenol E dicyanate ester Huntsman Kessler (2012) [39]

EX1510 Bisphenol E cyanate ester TencateTechnologies

Badrinarayan et al.(2012)

Primasett METHYLCy Tetramethyl bisphenol Fdicyanate ester

Lonza Kessler (2012) [39]

AroCy XU 366 Bisphenol M dicyanate ester Huntsman Kessler (2012) [39]

Primasett DT 4000 Dicyclopentadienyl bisphenolcyanate ester


Primasett PT Novolac cyanate ester phenolictriazine (PT)


AroCy XU 371 Novolac cyanate ester phenolictriazine (PT)

Huntsman Kessler (2012) [39]

(a) (b)

Figure 11.19 Use of cyanate ester-based materials in electronics and microelectronics [127,128].


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