Cyanate ester resins

The third resin in the series of high-temperature thermosetting resins is cyanate ester resin or polycyanurates. The use of cyanate ester resin is limited to aerospace and defense application due to their high cost. Aryl cyanate monomer was first synthesised by Grigat and co-workers [191] and commercial production of cyanate ester (CE) resin started in the mid-1980s. CE resins are characterised by two or more cyanate functional groups (-0-C=N-) with an aromatic or cycloaliphatic backbone. 

CE resins are available in liquid, semi-solid or low-melting solid form. CE resins contain cyanate monomers or a prepolymer, which is prepared by a controlled 

Nair and co-workers [203] reported polycyanate esters of an imide-modified novolac of different maleimide content. The resins underwent a two-stage independent thermal curing through trimerisation of the cyanate groups, as well as addition polymerisation of maleimide moieties. On heating, cyanate esters were transformed into an imido-phenolic-triazine network polymer. The cured resins exhibited a higher initial decomposition temperature compared with the cured maleimide novolacs. However, the thermal stability was found to be inferior to the conventional phenolic-triazine resin. 

Fig-1- Service temperature and toughness of various thermoset matrix polymer systems 

Property Epoxy Phenolic Toughened BMI Cyanate ester 

Reports on the use of cyanate ester resins in aerospace structures and related applications are scanty. Practically all the literature pertaining to their trials for potential aerospace applications are in the form of patents. All indications are 

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Note X=in-plane 0°, Y=in-plane 90°, 2 = through thickness, V,=fibev volume fraction, M=Tnat VGCF, H=hybrid VGCF, S=short staple VGCF, P=P-55 fiber, E=cpoxy, CER=cyanate ester resin. 

Note UTS=ullimate tensile strength, vokimc fraction, M=mat VGCF, H= CER-cyanate ester resin. 

Novolac hydroxyl groups reacted with cyanogen bromide under basic conditions to produce cyanate ester resins (Fig. 7.41).105,106 Cyanate esters can thermally crosslink to form void-free networks, wherein at least some triazine rings form. The resultant networks possess high s, high char yields at 900°C, and high decomposition temperatures.105... 

Novolac resins containing cardanol moieties have also been converted to cyanate ester resins.107 The thermal stability and char yields, however, were reduced when cardanol was incorporated into the networks. 

Reghunadhan Nair, C. R, Mathew, D. and Ninan, K N. Cyanate Ester Resins, Recent Developments. Vol. 155, pp. 1-99. 

The fluorine content, density, critical surface energy, glass transitions, thermal expansion coefficient above and below the glass transition, and 300°C isothermal thermogravimetric stabilities of the fluoromethylene cyanate ester resin system with n = 3, 4, 6, 8, 10 are summarized Table 2.2. Also included for the purpose of comparison are the corresponding data for the aromatic cyanate ester resin based on the dicyanate of 6F bisphenol A (AroCy F, Ciba Geigy). 

The fluorine content for the fluoromethylene cyanate ester resin system is clearly significantly greater than that for AroCy F and also significantly less than that for PTFE (76.0%). Correspondingly, the density reflects the fluorine content. As a homologous series, the contribution of incremental CF2 units can be quantified. From the density data, the volume equivalent of 41. 0 A1 /CF, structural unit in an amorphous thermoset matrix has been determined.8... 

The surface properties are of particular interest for composites and coatings. The n = 6 monomer will wet Teflon, and PTFE filled composites can be prepared. The critical surface tension of wetting for the fluoromethylene cyanate ester resin series has been determined from contact-angle measurements on cured resin surfaces. As indicated in Table 2.2, it parallels the fluorine composition and begins to approach the PTFE value of 18 dyn/cm. 

As indicated above, the dielectric properties are the chief consideration for applications of these fluoromethylene cyanate ester resins. As such, processing as coatings for microelectronic applications and as nonstructural castings and composites for radome-related applications is being investigated. In this respect the n = 6 system has been identified as the best compromise for synthesis, properties, and processing. Results in this section pertain only to this member of the series, which is currently under evahation for the above applications.13... 

I. Hamerton (ed.), Chemistry and Technology of Cyanate Ester Resins, Chapman and Hall, Glasgow (1994). 

Cyanamides, 3 157-171 23 107 economic aspects, 3 162-163 health and safety factors, 3 163-164 manufacture, 3 161 shipping and handling, 3 162 specifications and analysis, 3 163 Cyanate ester curing agents, 20 411 Cyanate ester resins, production of, 23 769 Cyanation, 9 313 Cyanazine, 23 321-322 Cyan couplers, in chromogenic chemistry, 19 252-253 Cyan dyes... 

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