Lap shear bond strength of thermoplastic polyimides and copolyimides

Adhesion properties of various thermoplastic polyimides and copolyimides were studied in order to develop suitable heat resistant adhesive film with excellent hot-melt processability under moderate conditions and to discover the effect of polysiloxane units introduced as soft segments on the bond strength. The single-lap shear strength of the steel joints adhered with the polyimides and copolyimides was found to depend on the structure of the aromatic components: linear aromatic polyimides with multi-phenylene linkages and meta linkages, particularly those prepared from 3, 3', 4, 4'-diphenylethertetracarboxylic dianhydride and 3, 3', 4, 4'-benzophenonetetracarboxylic dianhydride, showed an increased lap shear strength. The introduction of polysiloxane segments into the polymide backbone increased not only the processability of adhesion, but also the lap shear strength, significantly. However, the shear strength was lowered at high temperature because the cohesive force of the polymer decreased with decreasing glass transition temperature. The adhesion strength was also influenced by such bonding conditions as pressure and temperature.