We studied the thermal behavior and curing process of a cyanate ester resin (Cy)/epoxy resin (EP)/thermal latent polymeric hardener system based on a phenol–amine salt (PA-hardener) and the resulting cross-linked structure. Additionally, the physical properties of the cured product were evaluated. As a result, the curing system of Cy/EP with the PA-hardener was found to have unique thermal behavior, and an adhesion process was conducted in which the functionality and reactivity of the cyanate ester resin were utilized. The system cured rapidly at approximately 80 °C and could be completely cured at 120 °C in 1 h because the PA-hardener dissolved in the resins at an elevated temperature to give free amines, which could readily initiate the polymerization of cyanate ester resins and epoxy resins. The glass-transition temperature (T g) of the cured products increased with increasing a triazine-rich cross-linked structure derived from the cyclotrimerization of the cyanate ester resin as the curing proceeded at a relatively low temperature. The Cy/EP/PA-hardener system was found to have a rapid curing process and excellent adhesiveness, and it afforded a cured product with heat stability.

We have developed the curing agents that have good storage stability for cyanate ester resins. It should be noted that these agents can be given rapid and efficient curing at low temperatures around 100°C. Even though the nucleophilicity was reduced by a phenol‐amine salts consisting of basic aliphatic amines and weakly acidic phenols, the curing reaction with the cyanate ester occurred immediately. It means that the control of the curing reaction with cyanate esters is not easy due to the equilibrium between phenol and amine. In order to overcome this difficultly cyanate esters reactivity, the molecular motions suppression by polymer was applied in addition to the phenol‐amine salts. The effect of the suppression for hardeners was studied in terms of the storage stability and reactivity to cyanate esters. It has been found out that PSM‐EPEDA composed of ethylenediamine‐epoxy adducts and novolac phenolic resin exhibits a large storage stability against cyanate esters by its effective suppression of molecular motions accompanied with efficient and rapid curing around 100°C.

Effect of urea derivatives (UDs) on volume shrinkage during the curing of bisphenol A diglycidyl ether (BAGE)/bisphenol E cyanate ester (DCBE)/thermal latent polymeric hardener based on phenol-amine salt (PAH) was investigated. The UDs with N H bonds and bulky substituents reduced the volume shrinkage effectively. The UDs were just dispersed in the compositions before the curing, whereas the UDs dissolved in the cured compounds by the collapse of the dense packing of UDs based on hydrogen-bonding interaction and/or π-stacking after the curing. Besides, the curing behavior and the structure of cured materials were examined by DSC and IR. The results revealed that the UDs hardly affected the curing of the DCBE/BAGE/PAH system.