Long-term moisture effects on hygroscopic properties, and thermal and transition behavior of PBI/PEEK blend and its composite with carbon fibers, graphite flakes and hexagonal boron nitride

Long-term moisture effects on, and related thermal and transition behavior, of PBI/PEEK blend and PBI/PEEK matrix composite with carbon fibers (CF), graphite flakes (Gr) and hexagonal boron nitride (h-BN) are investigated. The absorbed moisture in the compression-molded neat PBI and injection-molded neat PEEK exposed to an ambient environment is determined first. Specimens were dried in a temperature-controlled environment. Changes of density, weight and volume are obtained in air and in toluene. Relationship between weight gain and volume change is established for neat PBI by a volumetric swelling coefficient (β V ) and determined to be 0.570. Thermal behavior and transition temperatures of neat PBI, PEEK, PBI/PEEK blend and their composite in as-received and dried conditions are studied with a differential scanning calorimeter (DSC). The PBI/PEEK blend and its composite are exposed to ambient environment and taken from several locations away from the injection port from outer surface of the molded tube to determine moisture profiles. Density, weight and volume changes due to moisture absorption are obtained. The swelling coefficients of PBI/PEEK blend and its composite are determined to be 0.524 and 0.489, respectively. Moisture effects on thermal transition are investigated. Glass transition and recrystallization melting transition due to annealing are significantly affected by moisture absorption for both PBI/PEEK blend and composite. For glass transition, as much as10°C for the blend and 8°C for the composite were observed. For annealing transition temperature, the maximum difference was 6 and 5°C for the blend and the composite, respectively. However, the melting temperature was not affected.