A facile method for enhancing the lap shear strength between CFRP and stainless steel via manipulating the molecular structure of bismaleimide-based adhesives

A series of bismaleimide-based prepolymers via the Michael addition reaction of N,N’-4,4’-diphenylmethane-bismaleimide (BDM) and 4,4’-diaminodiphenyl ether (ODA) were prepared and used as adhesives for bonding CFRP/stainless steel, which present excellent performance. The manipulating mechanism of the molar ratio on the structure-performance relationship was revealed. Stronger interaction of macromolecular chains (i.e. hydrogen bond force) becomes increasingly obvious with increase of the molecular weight (molar ratio of BDM/ODA decreases), evidenced by FTIR peak shift of characteristic functional group in macromolecular chains. The optimal BDM/ODA molar ratio of bismaleimide-based prepolymers is 6:5, on which lap shear strength (LSS) reaches a maximum value of 20.59 MPa; though the thermal stability (T_5d = 301 °C) and compressive strength (123.94 MPa) are not the best amongst these prepolymer adhesives, it really meets the requirement for high-temperature resistance as adhesive for bonding CFRP/stainless steel; it is the manipulating of molecular weight that achieves a balance between the cohesive strength and interfacial adhesion strength, which contributes to the highest LSS value (20.59 MPa) and testified by fracture morphology of the bonding interface. This is a facile and low-cost method for enhancing the lap shear strength between CFRP and metal substrate.