Bisphenol E cyanate ester as a novel resin for repairing BMI/carbon fiber composites: Influence of cure temperature on adhesive bond strength

The increasing use of polymer matrix composites (PMCs) in structural applications created demand for advanced repair techniques to fix internal delaminations in PMCs. One of a variety of repair techniques is injection repair, which involves injecting a low viscosity resin directly into the damaged area and subsequently curing the resin to heal the damage. In this study, bisphenol E cyanate ester (BECy) was investigated as a potential resin for injection repair of bismaleimide-carbon fiber based composite panels for aircraft. Temperature sensitive repair applications required a technique that avoided the high temperature post-cure of the injection repair resin. Modulated differential scanning calorimetry (MDSC) experiments were used to examine the degree of cross-linking and the glass transition temperature (Tg) of under-cured injection repair resin. The chemistry of cross-linking in under-cured BECy was studied by Fourier transform infrared spectroscopy (FTIR). Lap shear tests of the under-cured injection repair resin on composite substrates revealed the influence of change in isothermal under-cure temperature on the bond strength. Temperature dependent dynamic mechanical analysis disclosed the significance of sub-Tg relaxations on the adhesive properties of the under-cured resin. Post-fracture surface analysis of the lap shear specimens, performed using Scanning Electron Microscope (SEM) micrographs, indicated a mixed mode of fracture in the form of a combination of resin and resin–composite interface failure.