Polycyanate esters: Science and applications

The search for advanced, high performance, high temperature resistant polymers is on the rise in view of the growing demand for polymer matrix composites that are to meet stringent functional requirements for use in the rapidly evolving high-tech area of aerospace. Cyanate esters (CEs) form a family of new generation thermosetting resins whose performance characteristics make them attractive competitors to many current commercial polymer materials for such applications. The chemistry and technology of CEs are relatively new and continue to evolve and enthuse researchers. The CEs are gifted with many attractive physical, electrical, thermal, and processing properties required of an ideal matrix resin. These properties are further tunable through backbone structure and by blending with other polymer systems. The structure-property correlation is quite well established. Several new monomers have been reported while some are commercially available. The synthesis of new monomers has come to a stage of stagnation and the present attention is on evolving new formulations and processing techniques. The blends with epoxy and bismaleimide have attracted a lot of research attention and achieved commercial success. While the latter is now known to form an IPN, the reaction mechanism with epoxy is still intriguing. Extensive research in blending with conventional and high performance thermoplastics has led to the generation of key information on morphological features and toughening mechanisms, to the extent that even simulation of morphology and property has now become possible. Despite the fact that the resin and its technology are nearly two decades old, the fundamental aspects related to curing, cure kinetics, reaction modeling, etc. still evince immense research interest and new hypotheses continue to emerge.

Cyanate Ester Resins, Recent Developments

Although they are relative newcomers to the composites industry, cyanate ester (CE) resins are enjoying unprecedented success in certain applications. Several major space and radome manufacturers have qualified CE resins despite the extensive database on epoxies and the inherently conservative nature of the industry. Increasing demands on the materials used in these areas have stimulated the use of CE resins over other more conventional polymers. The aim of this review is to bring to the reader’s attention the more recent developments in the processing, toughening, properties and applications of cyanate esters, an exciting, versatile and important class of resins.

Recent Technological Developments in Cyanate Ester Resins

A unique hybridized intumescent flame retardant (hIFR), of which expandable graphite (EG) is chemically coated by organic chains containing phosphorous and nitrogen elements, was synthesized and characterized. Based on the successful synthesis, hIFR was used to develop modified cyanate ester (CE) resins with super flame retardancy. With the addition of only 5 wt% hIFR into CE resin, the peak and total heat releases significantly reduce to values that are only 32.3 and 23.1% of that of CE resin, respectively; meanwhile the fire performance index and limited oxygen index increase about 2 and 1.4 times. Besides, both smoke and carbon monoxide releases are remarkably reduced. These attractive data are much better than those of the modified CE resin with 5 wt% EG, clearly demonstrating that hIFR is a super flame retardant. Besides the traditional investigations on the structures of chars and cone calorimeter tests, an intensive study on the thermodegradation kinetics was carried out to reveal the mechanism of the outstanding flame retarding performance of the hIFR/CE resins. Different from EG, the unique structure of hIFR provides multi-effects on improving the flame retardancy, they are taking part in the structural formation of a cured network, increasing the thermal stability during the whole process of degradation, and strengthening the ability to form a thermally stable and condensed barrier for heat and mass transfer. These attractive features of hIFR/CE resins suggest that the method proposed herein is a new approach to prepare very effective flame retardants and corresponding super flame retarding resins.

A novel inorganic–organic hybridized intumescent flame retardant and its super flame retarding cyanate ester resins

Novel phosphorus-containing hyperbranched polysiloxane and its high performance flame retardant cyanate ester resins

The role of metal catalysts on the curing characteristics of bisphenol A-based cyanate ester resin systems was investigated using Fourier transform infrared spectroscopy, differential scanning calorimetry, and rheological measurements. Bisphenol A dicyanate was cured with the octoates and naphthenates of zinc and manganese and with cobalt acetylacetonate at concentrations ranging from 0 to 750 ppm metal. Nonylphenol at concentrations between 0 and 8 phr was used as the cocatalyst. The results show that at metal catalyst concentrations less than 100 ppm and nonylphenol concentrations less than 4 phr, a maximum glass transition temperature of 250 to 260°C is obtained, irrespective of the type of catalyst used. For samples cured without nonylphenol, the maximum Tg was 298 to 302°C. At high catalyst concentrations, the Tg decreases with increasing catalyst concentration for the zinc catalysts, whereas no concentration effect was observed for samples cured with either manganese or cobalt. Nonylphenol was effective as a cocatalyst in achieving high conversions; however, the Tg at full conversion was found to decrease with increasing nonylphenol levels.

Analysis of the curing behavior of cyanate ester resin systems

The kinetics of cyclotrimerization and the thermal stability of bisphenol Abased cyanate ester resin systems were determined using Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravimetry. The bisphenol A dicyanate was cured with 4 phr nonylphenol and with the octoates of zinc and manganese, and cobalt acetylacetonate at concentrations ranging from 0 to 750 ppm metal. An empirical rate law was used to predict the cyanate concentration profiles. The observed reaction rate showed a first-order dependence on the initial metal concentration and a second-order dependence on the cyanate concentration in the kinetically controlled regime. For the uncatalyzed systems, the kinetics was described by a second-order autocatalytic model. The thermal stability of the network was found to be dependent on the catalyst concentration for the zinc catalysts. For the samples cured with manganese, no effect of concentration on the thermal stability was observed.

A. Osei-Owusu

Papers

Analysis of the curing behavior of cyanate ester resin systems

Catalysis and kinetics of cyclotrimerization of cyanate ester resin systems