CE, cyanate ester

Fig. 17. Cloud-point curve (CPC) of the initial CE-NFBN system. Full lines represent CPC and sinnodal curve (S) fitted using the Flory-Huggins equation (CE = cyanate ester, NFBN = non-fiinctionalized poly butadiene-acrylonitrile copolymer) (Reprinted from Polymer, 36, J. Borrajo, C.C Riccardi, R.JJ. Wilhams, Z.Q. Cao, J.P. Pascault, Rubber-modified cyanate esters thermodynamic analysis of phase separation, 3541-3547, Copyright (1995), with kind permission from Butter-worth-Heinemann journals, Elsevier Science Ltd, The Bralevatd, Langford Lane, Kidlington 0X5 1GB, UK)...

Cyanate esters (CEs) are formed in excellent yields by the reaction of the corresponding phenols with cyanogen halide [34]. The reaction scheme is shown in Scheme la. The reaction is usually carried out in solution, in the presence of a tertiary amine as the acid scavenger at very low temperatures. Since the trimer-ization reactions of cyanate esters are highly prone to catalysis by spurious impurities, the most difficult aspect of cyanate ester synthesis is their scrupulous purification. Low molar mass esters are purified by distillation or recrystallization. Polymeric cyanates are purified by repeated precipitation in non-solvents such as water, isopropanol, etc. While distillation and recrystallization lead to pure materials, the precipitation method for polymeric cyanates is not always conducive to obtaining pure materials. A recent patent application claims a purification method for polymeric cyanates based on treatment with cation and anion exchangers [35]. 

Researchers have conducted a number of studies highlighting the synthesis of CEs, reaction conditions including solvents and catalysis [36-38], etc.,but most of the details are covered by patents. The solvents selected include acetone [39], methyl isobutyl ketone [40], dichloromethane [41], etc., and acid scavengers chosen mainly include triethyl amine and pyridine [39, 41]. The use of tri-alkylamines often leads to the formation of dialkyl cyanamides as side products, which are known to catalyze the reactions of cyanate esters, reducing the yield. The formation of these impurities depends on the mode and rate of addition of reagents, reaction temperature, etc. Factors like stirring rate, dropping rate, and... 

A variety of CEs with tailorable physico-chemical and thermo-mechanical properties have been synthesized by appropriate selection of the precursor phenol [39,40]. The physical characteristics like melting point and processing window, dielectric characteristics, environmental stability, and thermo-mechanical characteristics largely depend on the backbone structure. Several cyanate ester systems bearing elements such as P, S, F, Br, etc. have been reported [39-41,45-47]. Mainly three approaches can be seen. While dicyanate esters are based on simple diphenols, cyanate telechelics are derived from phenol telechelic polymers whose basic properties are dictated by the backbone structure. The terminal cyanate groups serve as crosslinking sites. The polycyanate esters are obtained by cyanation of polyhydric polymers which, in turn, are synthesized by suitable synthesis protocols. Thus, in addition to the bisphenol-based CEs, other types like cyanate esters of novolacs [37,48], polystyrene [49], resorcinol [36], tert-butyl, and cyano substituted phenols [50], poly cyanate esters with hydrophobic cycloaliphatic backbone [51], and allyl-functionalized cyanate esters [52] have been reported. 

Although a large variety of such experimental cyanate esters have been reported, few have achieved commercial success. Some of these have been listed in Table 2 along with their key properties. The properties are very much dependent on the backbone. Thus, the experimental CE resin XU.71787 developed by Dow... 

As mentioned earlier, the cyanate-epoxy systems find many commercial applications. Many multifunctional formulations, principally based on CE/epoxy, have been claimed. Generally, the properties of epoxies are improved on co-cur-ing by cyanate ester and the blend is more cost-effective than cyanate alone. The applications of these blends, as covered by patents, has been referred to in a review by Penczek and Kaminska [184]. Most of their applications are in copper clad laminates [185-191], fire resistant formulations [192], aircraft structures [193], and in semiconductor devices [194-196]. Due to the commercial significance of the blends, the information on the epoxy-cyanate systems is still covered by patents and some of the most relevant and recent ones have been referred to in this review. 

Cyanate ester (CE) resins are the key monomers for a new type of high-performance polymer. They were developed by Hi-Tek Polymers during the 1980s, then Rhone-Poulenc, and now Ciba-Geigy. The polycyclotrimerization... 

Other thermosetting polymers are cyanate esters (CEs), benzoxazines, PU acrylates, bismaleimides (BMIs), dicy-clopentadienes (DCPDs), diallyl phthalates (DAPs), etc. Formulations based on these polymers are used for specific applications where their particular properties are required. For example, DAP has long been the material of choice for electrical components where long-term reliability is required. 

The matrix is the weakest component of the composite determining the allowed maximum stress and the maximum service temperature. Great efforts have been undertaken to develop thermosetting polymers with increased temperature resistance. Cyanate-esters (CEs), BMI, and polyimides are the actual thermosetting matrices with higher thermal resistance. 

The cyclotrimerization of a cyanate ester (4,4 dicyanato-1,1 diphenylethane, CE) (Fig. 16), in the presence of a butadiene-acrylonitrile random copolymer terminated by non-functional groups (NFBN), has been analyzed [89]. 

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. 

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