Effects of core-shell and reactive liquid rubbers incorporation on practical adhesion and fracture energy of epoxy adhesives

Epoxies offer many advantages in structural applications compared with other adhesives. However, their highly cross-linked structure leads to brittle materials, with low toughness and poor impact resistance. Usually, core-shell and reactive liquid rubbers are incorporated into industrial epoxy adhesives to overcome these limitations. Although important studies have been published in this field, it still lacks in the literature a comprehensive comparison between the effects of incorporating each type of rubber into the same epoxy matrix. Additionally, these reports usually focus on the behavior of the adhesive bulk, neglecting the effect of tougheners inclusion on the adhesively bonded joints fracture energy. Aiming to fill this gap, in this work, we prepared epoxy adhesives reinforced with core-shell rubber particles (CSR) or with a reactive liquid rubber (carboxyl-terminated polybutadiene-co-acrylonitrile-CTBN), at constant rubber contents in both formulations. Then, the toughened adhesives were investigated with respect to their thermomechanical behavior, as well as their adhesion performance. Bulk properties were characterized by uniaxial tensile tests and dynamic mechanical analysis, while the practical adhesion was investigated with single-lap shear and peeling tests. Finally, we assessed the adhesively bonded joints fracture energy in a customized experiment. The results showed that CTBN incorporation improved the adhesively bonded joints fracture energy by almost 816%, while CSR increased it by 550%. However, CTBN decreased the T-g of epoxy adhesives from 160 up to 130 degrees C, while CSR incorporation revealed no significant influence on T-g. Practical adhesion was increased by incorporation of both tougheners. In short, CTBN-containing samples revealed to be more efficient, even in smaller concentrations.