Adhesive strength of partially imidized thermoplastic polyimide films in bonded joints

Thermoplastic polyimides, when used as adhesives in fully imidized form, exhibit a viscoealstic solid-like behavior and require high pressure to achieve proper wetting of the adherend surface. In this study adhesive strength of partially imidized thermoplastic polyimide films in bonded joints is investigated when processed at low pressures. An amorphous polyimide (ODPA/3, 4′-ODA/PA) and a semi-crystaline polyimide (BPDA/3, 4′-ODA/PA) are studied and their adhesive properties are evaluated. The polyimide films are cast from poly(amic acid) (PAA) solution and are thermally imidized at various temperatures. Degree of imidization is determined using FTIR spectra. DMTA analysis is carried out to measure and compare thermal properties and to design final cure cycle. Lap shear strength (LSS) is measured to compare the adhesive strength of single lap joints prepared with steel adherends. Fractured surfaces are analyzed using SEM and optical microscope. LSS is also measured at high temperatures to investigate the temperature resistant capabilities of bonded joints. Solvent resistance was evaluated by measuring LSS of bonded joints after exposure to solvent attack. It was found that bonded joints prepared with partially imidized films exhibit higher adhesive properties as compared to joints prepared with direct application of PAA or powder oligomers. Semi-crystalline polyimide exhibited higher LSS, better retention of LSS at high temperature and also after exposure to solvent attack as compared to amorphous polyimide.