Studies On The Modification Of Cyanate Esters And Their Composites

Polymeric composites have received more and more attentions for their advantages in mechanical properties, lightweight and easy processing. They have been widely used in aerospace structures, aircrafts, dielectrics, some other government issues and civil industry products in recent years. Cyanate esters are one kind of the most important thermosets matrixes in composites. They had been studied from 1970s. Their unique chemical structures endow them with good perporties, such as the high performance of mechanical properties, low water up-takes, good thermal and high temperature performance, perfect dielectric properties and low out-gassing during curing and so on. And they have been used in composites, adhesives, and electrics. However, like most other thermosets, they have suffered the drawback of brittleness. At the same time, reports on cyanate esters are all focused on bisphenol A dicyanate ester (BADCy). According to these, the following aspects in cyanate esters have been studied in this dissertation.1) Copolymers based on dicyclopendiene bisphenol dicyanate esters (DCPDCE) and epoxy E51 has been prepared. Curing kinetics, thermo-decomposition kinetics, mechanical properties of the cured resins and glass fiber-reinforced composites of the copolymers have been systematically studied. It is indicated that E51 acted as a catalyst for the forming of triazine rings during the low temperature (＜50℃) and then reacted with triazine rings to forming oxalindone rings with the temperature increasing. On the other hand, epoxy can also be a diluted substance, and if the dosage of E51 were large enough, the catalysis effects would be smaller then dilution effects. These conclusions were proved by the kinetics studies of the copolymers, the kinetic parameters of the curing reactions of DCPDCE/E51 copolymers have little changes with the additions of E51. E51 can effectively improve the mechanical performance of DCPDCE, the impact strength and flexural strength can be improved by 29.7% and 38.4%, respectively, if 5wt% of E51 was used. Toughening mechanism was discussed according to the micrographs of the failure specimens. However, thermal properties of the cured resin were weakened with the introduction of E51. The more E51 used in the systems, the lower the Tg values from DMA analysis and maximum decomposition temperature from TGA analysis are, although the mechanism of thermal decomposition of the different systems are the same one. E51 can improve the mechanical properties of glass fiber reinforced DCPDCE composites, but is not good for the electric performance. Therefore, the usage of E51 in the copolymers should be controlled between 5wt% and 15wt%.2) Carboxyl-terminated liquid butadiene-acrylonitrile was applied in toughening the mechanical performance of BADCy and DCPDCE. The reactions, performance of cured resin and fiber-reinforced composites of the two systems have been studied. In the two systems, CTBN can all act as a catalyst and can effectively improve the mechanical properties. Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were employed to show the effects of CTBN on the curing behavior of BADCy and DCPDCE resins, indicating that the additions of CTBN have large influence on the curing of cyanate esters. The perfect usage of CTBN in the BADCy/CTBN system would be 15wt%, while it is 2wt% of the DCPDCE/CTBN systems, reasons of these lie in the different chemical structures of BADCy and DCPDCE. The hole shearing bending theory can well explain the toughen mechanism in the two systems.3) Poly vinyl pyrrolidone (PVP(K30)) / Bisphenol A Dicyanate ester (BADCy) blends were fabricated to increase the toughness of BADCy by blending processing in this paper. Curing parameters were determined by gelation time curves and DSC of the systems. FTIR and DSC data were employed to show the curing behavior and kinetics of the systems. Mechanical properties of the cured resin had been improved rapidly with the increasing of PVP(K30) at low mass fraction, but would decrease when mass fractions of PVP(K30) were higher than 15wt%. Scanning electron micrograph (SEM) was applied to show the microstructures of the cured matrixes. Mechanical properties can be improved by 2.36 times while 8wt% of PVP(K30) were used. And the hydrothermal aging of the cured blends was carefully studied. It can be concluded that the addition of PVP(K30) can improve the toughness of BADCy greatly with little loss of other properties.