Siloxane Modification of Bismaleimide Resins

Abstract

Polybismaleimides have high thermostability, but they are very brittle materials. Introduction of spacers such as aromatic diamines by addition onto the maleimide double bonds gives after crosslinking of the remaining double bonds, a polyaminobismaleimide network with greatly reduced brittleness. On this principle are based KERIMID resins and KlNEL moulding powders developed by Rhone Poulenc in the early 70’s. Continuing the study on toughening of bismaleimide resins, a novel concept based on their siloxane modification has been developed and it appears very promising for the formulation of hot-melt resins with tailored reactivity and easy processability by filament winding, vacuum bag moulding or injection. Thus obtained carbon fibers reinforced materials (CRFM) show good thermomechanical properties even after 40 hours ageing in boiling water or ≥1000 hours in air at 250°C (more than 50% retention of the initially measured properties). Basically this type of resin is formed from bismaleimides, diphenylsilanediol and a reactive solvent. An imidazole type catalyst is added to have the desired reactivity of the system. Curing of this resin is achieved according to an epoxy type cycle and gives an opaque material with Tg=353°C. This approach of polybismaleimide toughening is new, in comparison to those based on chemical linking of siloxanic units onto the polyimide chain and appears very promising for improving physical and thermomechanical properties of bismaleimide resins.