The Role of Hydroxy Substituent Position in Hydroxybenzonitrile Homologs on the Curing Behavior and Mechanical Properties of Cyanate Ester Resins

ABSTRACT The application of cyanate esters has been greatly limited by their high curing temperatures, which induce residual thermal stress and subsequently degrade their mechanical properties. In this study, hydroxybenzonitrile homologs (HBN x ) were developed to reduce curing temperature of bisphenol E cyanate ester (BECE) while improving their mechanical properties. It was found that the position of hydroxy (‐OH) substituent in HBN x had a significant impact on the curing temperature of BECE. Among the homologs, ortho‐hydroxybenzonitrile (HBN 2 ) demonstrated the highest reactivity, as indicated by quantum chemical calculations and confirmed by differential scanning calorimetry analysis. As a result, the curing temperature of BECE was effectively reduced from 250°C to 140°C with the addition of 15 wt% HBN 2 . Additionally, the addition of HBN x improved the mechanical properties of cyanate ester resins and their composites reinforced by quartz fibers (GF). Specifically, the mechanical properties of HBN x /BECE castings remained at high levels, with impact strength, tensile strength, and flexural strength measured at 38.6 kJ/m 2 , 94.5, and 100.0 MPa, respectively. Furthermore, the mechanical properties of GF/HBN x /BECE composites exhibited an overall improvement, with the maximum enhancements in short‐beam shear strength, tensile strength, and flexural strength reaching 78.0%, 27.3%, and 45.1%, respectively. Nevertheless, the hydroxy substituent position in HBN x had a relatively minor impact on these mechanical properties. This study presents a promising strategy for synergistically enhancing both the processability and mechanical properties of CE resins and their composites in a cost‐effective manner.