November 2023

CLS system: Rapid curing systems with cyanate ester
resins.

Junji Ueyama, Ryo Ogawa,New curing systems for epoxy resins

In general, curing of epoxy resins by amines, which are typical thermosetting resins, takes time.

This is due to the reaction mechanism and reaction rate between the epoxide and curing agent 3.

Epoxy resins and curing agents are often used as one-component types for curing at hightemperature and as two-component types mixed just before use for curing at low/ambient
temperature, mainly from the viewpoint of storage stability. In order to achieve rapid curing with
one -component type, the control of the stability control is most important technology.

We call the epoxy resin adhesives used in our laser ultra-rapid curing system the "CLS system"(Curing system of Latent hardener and Specialty epoxy resin). The CLS system enables thermoset

ultra-rapid curing adhesives using a new compounding technology that combines original epoxy
resins and latent hardeners with a highly reactive cyanate ester resins.

1. Cyanate ester resins and curing agents

Cyanate ester resins have been researched and developed since the 1950s as thermosetting

resins with high heat resistance, low dielectric properties, mechanical strength and chemicalresistance, based on the formation of a triazine structure by cyclo-trimerization. Cyanate ester

resin was derived from phenol and developed in 1976 as Triazine A (bisphenol A dicyanate ester)for printed circuit boards. However, due to its purity, Triazine A had problems regarding moistureresistance in the process. Since the 1980s, various phenol-based high-purity cyanate ester resins

have been produced. After many years of research and development, cyanate ester resins havebeen used in space, aerospace, high-gain antennas, High-Speed Civil Transport (HSCT) andSuperconducting Supercollider taking advantage of its characteristics since the 1990s 2.

In general, the trimerization of allyl cyanate ester resins occurs slowly at around 170-200 ºC

without catalyst, so that high temperatures and/or long time are required for complete curing.

Therefore, in general, metal-catalysts such as cupper acetylacetonate are employed to complete

curing. On the other hand, since metal-catalysts are used by melting in cyanate ester resins, thecatalyst cannot be melted immediately during curing, so the curing products are with catalyst
residue. Moreover, the particles formed by catalyst residue in cured resins are not preferred.

Therefore, an alkylphenol such as nonylphenol as a co-catalyst is often employed. It also works as
the solvent for metal-catalyst, thus, it could promote further cyclo-trimerization to cure the resin
completely at about 170 ºC in a short time 3,4. Some studies on more efficient curing catalysts,

such as metal species and hydrogen-donating co-catalysts to expand the application process havebeen reported 5-7. A recent study by Palmese et al. reported the use organic ionic liquids without

metal as curing agents 8. On the other hand, although cyanate ester resins are a high-performanceresin with potential, there are few reports on low-temperature curing and one-component curing
because of difficultly of reaction control 9.

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Meanwhile, polymerization reactions involving addition reactions of cyanate ester resins arehighly reactive and can react with acidic or basic compounds. For example, trimerization

reaction with weakly acidic phenols, via an intermediate imino carbonate. Reactions witharomatic amines, which are basic compounds, are known to give triazines substituted withamino groups via isourea intermediates 10 (Figure 1). Especially, Reactions with aminecompounds, which are basic compounds, are intense and are often accompanied by a large
reaction heat upon mixing. Hence, when cyanate esters andmust be controlled and cured.
R1

R1

OCN + HOOCN + H2N
R1, R2R2

R2= Aromaticamines are reacted, the reaction

NHR1OO

R2R2N
HONRNHR1NR
NRor -NHR
R= -OR1
2Figure 1 Reaction of cyanate esters with phenols and amines.

2. One-component cyanate ester resin compositions withlatent curing agents

Cyanate ester resins react violently with bases such as aromatic amines and are particularly

dangerous when reacting with aliphatic amines. Therefore, the CLS system is controlled thereaction and curing temperature nucleophilicity of amines and acids, and the polymer effect as

the molecular motions suppression. In general, acids and bases are in equilibrium, and theequilibrium constant is determined by the acid and base selected and varies depending on theenvironment and temperature. For example, the equilibrium state of amine and phenol wasreported in detail by Zundel et al. 11, 12 (Fig. 2). Here, when benzylamine and p-cresol are mixed at
equal mol, the equilibrium state product is obtained as a liquid. On the other hand, equal mol

mixtures of phenylethylamine (m.p. 48 ºC) and p-cresol (m.p. 32 ºC) give salts having melting

point at 76 ºC. When these salts are mixed with cyanate ester, it can be confirmed that they react

instantly with a significant exothermic. The reaction of aromatic amines, phenol and cyanate ester
resins was also described by Jensen et al. 13. In other words, the formation of salts by simple
acid/base reactions is not sufficient. Therefore, by also utilizing the control of molecular motion

by polymers, it will be possible to control the reaction of amines, phenols, and cyanate ester resins.

The latent hardener has a Tg of around 80 ºC (Fig. 3) and does not react when mixed with liquidbisphenol E cyanate ester resins, as indicated at the characteristic peak around 2200 cm-1

originating from the stretching vibration of the -OCN group (Fig. 4). The DSC measurement resultsof the mixture of the latent hardener and the bifunctional cyanate ester resin are shown in Fig. 5.

Essentially, cyanate ester resins and amine compounds react instantaneously with significant

exotherm as soon as they are mixed. However, we have succeeded not only in controlling the©2023 ADEKA Corporation

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reactivity of the curing agent by latent technology, but also in making the hardener react at themoment it melts.

NH2 + HOR2

R2R2NH2HONH3R2R2OR2

Figure 2 Equilibrium reaction between phenol and amine.

Calorific value (W/g)

-0.1
-0.2
-0.3
-0.4
-0.5

507090110
Temperature (ºC)

130

150

Figure 3 DSC curve of latent hardener.

Intensity (a.u.)18
1383-23600

3100

2600

2100

1600

1100

600

Wavenumber (cm-1)
Figure 4 IR spectra for mixture of cyanate ester resins and latent hardener.

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Carforicvalue (W/g)

18141062-250

7090
110
130
Temperature (ºC)

150

Figure 5 DSC curve with mixture of bisphenol E cyanate ester and the latent hardener.

3. Curing Temperature Control and Curing Process withLatent hardener in CLS Systems

The CLS system consists of a combination of epoxy resin, cyanate ester resin and latent hardener.

As described above, our latent hardener technology enables this one-component system, and thecuring temperature can also be controlled by the amine species of the latent hardener (Figure 6).
The curing temperature of latent hardener using highly reactive aliphatic amines is approximately

70 ºC, that of simple aliphatic amines is approximately 90 ºC, and that of aliphatic amines modified

Exotherm →with controlled structure is approximately 120 ºC.

2060100140180

220260300 ºC

Figure 6 DSC curves with each latent hardener ;solid line: highly reactive aliphatic amine
dotted line: simple aliphatic amine
dashed line: structure-controlled aliphatic amine
We are also investigating the curing process of the CLS system by using corresponding

monofunctional model compounds. Since cyanate esters and epoxides do not react even at 180ºC, the expected reaction is between amines and cyanate esters or epoxides, although cyanate
esters react much more rapidly due to the difference in reaction rates. The detailed reaction
between cyanate esters and amines has only been reported by Bauer et al. for aromatic amines 12,©2023 ADEKA Corporation4

November 2023
but there are no detailed reports for aliphatic amines. When cyanate esters are in excess, most of
the reactions with aliphatic primary and secondary amines lead to trimerization via cyanate amine
adducts, according to an analysis of the elementary reactions of cyanate esters with amines (Figure7). The reaction with aliphatic amines is similar to that of aromatic amines, but the aliphatic amine

reacts more rapidly, especially when primary amines are used. Furthermore, cyanate-amine
adducts consisting of primary amines are unstable and dissociate in solution within a several hours(Figure 8). The reaction with tertiary amines is considerably slower than the reactions with primaryand secondary amines, but it has been shown to promote gradual trimerization of cyanate esters
at room temperature.

R1
R1
R1

OCN + H2N R2= Aromatic, R = AliphaticR2O

R1NH

2

R1ONROCN

NNHN
R

R= -OR1

R1
R1

OCN + HN

R1R2R2

R2OCN

R1NR22or -NHR

NONH

N

R2= Aromatic, R = Aliphatic

R1O

RNO
N

N

2

R1

R2Figure 7 Reaction of cyanate ester s and amines.

R1
R1
R1

OCN + H2N R2

R2= Aromatic, R = Aliphatic
2Disocciation

R1NHO

R1ON

OCN + R2 N CHNH

N+ R
1OH

Figure 8 Dissociation reaction of cyanate-amine adduct.
As a result of examining the reaction of epoxides with cyanate-amine adducts consisting ofsecondary amines, cyclized oxazolines and epoxy oligomers from adducts or phenoxides wereobserved (Figure 9). An interesting fact about this reaction is that the phenol generated by thethermal decomposition of the adduct activates the reaction of the adduct with the epoxide. As

shown in Figure 10, when equal mol of phenol is added to the reaction of this adduct with epoxide,
the reaction is significantly lower in temperature.

However, this reaction is minor in the reaction between cyanate ester and amine, and triazine

structure formation by trimerization is by far the predominant reaction. Several reactions between

the formed triazine and epoxide have been reported 14, 15, including the insertion reaction of thering-opened epoxide into the triazine, isocyanurate formation by isomerization, and finally©2023 ADEKA Corporation5

November 2023
oxazolidinone by reaction with the epoxide 16. Based on the above elementary reaction analysis
and previous reports, the curing process of the CLS system is shown in Figure 11, and its cross-linked structure is controlled by the ratio of components.

O
R2

N

R1

O

O

R3

R3R2NN

NHO

R2

R2

OONHR2N

R3OOO

R1O

R2O

R1OHO

R3R3

Figure 9 Reaction products of cyanate-amine adducts and epoxides.

Figure 10 DSC curves for mixture between cyanate-amine adduct and epoxide.
solid line: with phenol
dotted line: without phenol.
R1
R1
R1

OCN + H2N R2= Aromatic, R = Aliphatic
2R2NHO

R1O

N

R1OCN

R
N

NH very fast

RN
NR

R= -OR1
EPor -NHRNO

EPslow

2NO
NO
N
R2NO+EPO

NO

Ring open polymer

Figure 11 Proposed curing reaction process of CLS system.

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Conclusion

We have succeeded to control the high reactivity of cyanate ester resins and amine compounds,

which was conventionally considered to be difficult to control. Furthermore, we succeeded indeveloping the CLS system, a rapid curing system combining epoxy resin, cyanate ester resin andlatent curing agents, and found that it has a new curing behavior not seen before.

In the future, we expect to develop a variety of applications utilizing the CLS system.

©2023 ADEKA Corporation

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Bibliography

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2. Hamerton I., Chemistry and Technology of Cyanate Ester Resins, Blackie Academic andProfessional, Glasgow (1994)

3. Hi-Tech Polymers Inc. (1989) US Patent 4,847, 233
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5. Li, W. Liang, G., Xin, W. (2004) Polym. Int., 53 (11), 869-876
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9. Shimp, D. A. and Christenson, J. R. (1990) Plastics-Metals-Ceramics, ed. Hornfeld, H. L.,SAMPE, p. p. 81-94

10. Bauer, J., Bauer, M., (2001) Macromol. Chem. Phys., 202, 2213-2220
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12. Menger, F. M. and Barthelemy, P. A. (1996) J. Org. Chem., 61, 2207-2209
13. Jensen, K. A., Holm, A., (1977) In The Chemistry of Cyanates and Their Thio Derivatives, Part1, ed. Patai, S. Wiley, New York, 569-617

14. Bauer, M., Tänzer, W., Much, H., Ruhmann, R., (1989) Acta. Polym., 40 (5), 335-340
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16. Ohno, D., Kazuya, Z., Youji, K., Kazuyoshi, U., Hitoshi, H., (2016) Int. J. Org. Chem., 6, 117- 125©2023 ADEKA Corporation8