Although bismaleimide-based adhesives offer substantial improvement in high temperature capability in comparison to their epoxy counterparts, they do suffer a number of disadvantages. Firstly, although their processability can be considered a significant improvement over more traditional high temperature polymers they still do not compare directly with epoxy type processing. An approach which the Design Research Agency (DRA) is taking is to combine the high temperature properties of bismaleimides with epoxy resins. A series of compositions were manufactured, based on combinations of diglycidylether of bisphenol A, diphenylsulphone and bismaleimide. In the majority of cases the introduction of the epoxy to the bismaleimide increases the fracture toughness and the fracture energy with modest reductions in both the modulus and the glass transition temperature. A modest improvement in lap shear properties is possible, but the most important benefits will accrue at higher temperatures. It is hoped that the toughness of these systems can be improved by the incorporation of a second phase rubber.
By developing such adhesive systems it is possible to create tough high temperature adhesives which will withstand the sorts of service temperature and loads experienced by modern aerospace equipment.