Cyanate ester

Cyanate-ester adhesives are formed from resins that contain cyanate groups 

Catalysts may be the soluble salts or complexes of transition metals, for example, those of tin, copper, iron, or cobalt, or compounds that have an active hydrogen atom such as mercaptans, phenols, or amines. 

Modified eyelo-olefin thermoset (MCOT) resins are liquids that cure when heated at 150°C for a minimum of 30 minutes to form the general polymer structure shown in Fig. 3.14. The monomers cure by an addition polymerization and, as such, no water or other by-products are evolved. The molecular structures are essentially nonpolar which impart very low moisture absorption properties. For two formulations developed by Johnson-Matthey, water absorptions of 0.05% were recorded when exposed to 85°C/ 85% RH conditions for 168 hours. Only 0.1% water absorption was 

The cyclic olefin block is a rigid segment while the linear olefin is a soft (flexible) block. In Fig. 3.14, Rj and R2 comprise the nonpolar and polar portions, respectively, that control the amount of moisture absorption. The optimized value for R is 18% and for R2,3 6%. A molecular weight of 3,262 is reported to provide the best combination of adhesion, moisture resistance, and viscosity properties, 

Cyanate esters are a family of aryl dicyanate monomers that contain the reactive cyanate (-0-C=N) fimctional group. When heated, this cyanate functionality undergoes an exothermic cyclotrimerization reaction to form triazine ring connecting units, resulting in the formation of a thermoset polycyanate polymer. 

The cyanate ester monomers are available from low-viscosity liquids to meltable solids. Ciba-Geigy produces a series of cyanate esters imder the trademark AroCy L-10. Cyanate esters are used in manufacturing structural composites by filament winding, resin transfer molding, and pultrusion. 

Schweitzer. 2004. Corrosion Resistance Tables, Vols. 1-4, 5th ed.. New York Marcel Dekker. 

---

Currendy, epoxy resins (qv) constitute over 90% of the matrix resin material used in advanced composites. The total usage of advanced composites is expected to grow to around 45,500 t by the year 2000, with the total resin usage around 18,000 t in 2000. Epoxy resins are expected to stiH constitute about 80% of the total matrix-resin-systems market in 2000. The largest share of the remaining market will be divided between bismaleimides and polyimide systems (12 to 15%) and what are classified as other polymers, including thermoplastics and thermoset resins other than epoxies, bismaleimides, cyanate esters, and polyimide systems (see Composites,polymer-matrix-thermoplastics). 

Modified Bismaleimides. Bismaleknide resins may be further modified and blended with other thermoset resins or reactive diluents to achieve either specific end-use properties or processibiUty. Thermoset resins that can be used for modification are unsaturated polyesters, vinylesters, cyanate esters, and epoxies. 

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. 

Despite all precautions, urethanes can be used most effectively within certain thermal and oxidation limits. Outside these parameters, other adhesives, such as certain epoxies, cyanate esters, and other high-temperature adhesives, should be considered. 

Because of this continued emphasis on adhesive bonding technology development over the years, the airframes of modem front-line aircraft such as the B-2 bomber and the F-117 and F-22 fighters are largely structurally bonded advanced composites. They tend to be comprised of materials that are more advanced (expensive) than commercial aircraft such as carbon and boron fiber reinforcements with cyanate esters, bismaleimides, polyimides or other high-temperature resin matrices and adhesives. 

The cyanurate ring is formed hy the trimerization of the cyanate ester. 

Potamkfe-lmiiWGIass Phenol-formaldehyde Cyanate Ester/Glass... 

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. 

Figure 14.17 Cyanogen bromide can be used to activate a hydroxyl-particle to a reactive cyanate ester, which then can be used to couple amine-containing ligands.

Cyanogen bromide can be used to activate hydroxyl groups on particles to create reactive cyanate esters, which then can be coupled to amine-containing ligands to form an isourea bond (Figure 14.17). CNBr activation also can produce cyclic imidocarbonate groups, which are less reactive than the cyanate ester, but can form imidocarbonate bonds. The exact reactive species formed by the reaction is dependent on the structure of the hydroxylic support being activated (Kohn and Wilchek, 1982). 

High-temperature polymers Fluorinated cyanate ester 40... 

Figure 2.1. Fluoromethylene cyanate ester structures and curing reaction.

The NCO—CH2(CF2) CH2—OCN monomer series have been characterized by DSC, IR, H-NMR, 19F-NMR, 13C-NMR, and elemental analysis.8 Table 2.1 summarizes the characterization most pertinent to these cyanate ester monomers. The n = 5,1, and 9 members are missing. This is a reflection of the difficulty in obtaining the odd hydrocarbon diol precursors. The trend of a rapid melting point increase with increasing fluoromethylene sequence length is an indication that monomers with n > 10 will probably not be melt-processible since the onset of the cure exotherm in most purified monomers occurs at 200°C. 

The hydrolytic susceptibility of the fluoromethylene cyanate ester is greater than that of the aromatic cyanate ester. Direct contact with water results in measurable hydrolysis to the carbamate in a 24-h period for the former while the latter is unaffected.9... 

It has been established by a variety of techniques that aromatic cyanate esters cyclotrimerize to form cross-linked cyanurate networks.1 Analogously, the fluoromethylene cyanate monomers cure to cyanurate networks. In addition to the 19F-NMR spectra shown in Figure 2.3, evidence includes an up-field shift of the methylene triplet (1H-NMR, 0.21 ppm 13C-NMR, 9.4 ppm), the disappearance of the cyanate functional group (IR, 2165 cm4 13C-NMR, 111.9ppm) and the appearance of the cyanurate functional group (IR, 1580 and 1370 cm4 13C-NMR, 173.6 ppm).9 Typically, monomers are advanced to prepolymers by thermal treatment at 120°C or just above the melting point. The prepolymers are then cured at 175°C and are postcured at 225°C. 

The curing reaction can be carried out thermally or with the addition of a catalyst. The thermal cure is strongly influenced by impurities associated with the synthesis. The greater the degree of monomer purity, the more slowly the thermal cure proceeds. If the monomer is sufficiently purified, the cure rate can be predictably controlled by the addition of catalysts. As with the aromatic cyanate esters, the fluoromethylene cyanate esters can be cured by the addition of active hydrogen compounds and transition metal complexes. Addition of 1.5 wt% of the fluorinated diol precursor serves as a suitable catalyst.9 The acetylacetonate transition metal salts, which work well for the aromatic cyanate esters,1 are also good catalysts. 

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. 

---