Heating methods for bonding thermoplastics to aluminum alloy

The fabrication of composites from polyimides that involve condensation reactions often requires the depletion of volatiles (solvent and reaction by-products) prior to consolidation in order to achieve a void-free laminate. However, volatile removal during the B-staged period involves imidization that causes resin viscosity to increase and the composite processability to suffer. The residual processability becomes a serious issue, especially in the fabrication of large structural parts using an autoclave where temperature non-uniformity is commonly experienced. In this study, the residual processability of B-staged IM7/PETI (phenylethynyl terminated imide) prepreg was characterized using the Parallel-Plate Plastometer (PPP). Prepregs were B-staged at temperatures between 225 and 300°C with duration times between 0.5 and 4 hrs. The residual processability was defined by the percent deformation of the test specimen by the PPP. Significant reductions in the percent deformation were measured for specimens B-staged at 300°C for duration times longer than two hrs. B-staged specimens with poorer residual processability were also shown to exhibit lower densities and higher void contents. Because of the inherent limitation on specimen sizes, the PPP measurements were shown to over-estimate the residual processability of the B-staged prepregs. In conjunction with the PPP experiments, rheological measurements were also conducted. Different rheological behaviors were obtained and interpreted with the aid of thermal analyses, which revealed the crystallization behavior of PETI prepregs under different B-staged conditions and its effect on the residual processability.