Counter‐Intuitive Mechanical Degradation of QF / PI Composites due to Residual Solvent

ABSTRACT The use of solvent is a common strategy to improve the molding performance of PI‐based composites; however, the complete removal of solvent during processing is very challenging, and the residual solvent generally has an unidentified deteriorative effect on the overall mechanical properties of composites. In this paper, we fabricated different quartz fiber (QF) reinforced polyimide (PI) composites with qualitatively controlled content of remaining solvent via modified multi‐layer resin film infusion process (MLRFI) and autoclave assisted vacuum bagging (AVB) procedure. The mechanical results showed that the residual solvent shows a counter‐intuitive temperature dependency on the strength: at RT, voids dominated—flexural strength rose from 586.0 MPa (9.7 vol.%) to 685.6 MPa (6.3 vol.%); at 150°C, high‐residual solvent sample (V6.5‐RS4) had a larger flexural strength drop (94 MPa vs. 85 MPa of high‐void sample (V9.7‐RS3)); at 300°C, residual solvent dominated, with V6.5‐RS4 showing the lowest strength. Through analyzing the failure mode and crack distribution under different temperatures, a competitive deterioration effect induced by pore defects and residual solvent was revealed. At temperatures lower than the boiling point of residual DMAc solvent (164°C), pore defects show a decisive role on strength due to stress concentration and crack initiation. At temperatures higher than 164°C, the deterioration effect caused by the expansion and interface enrichment of residual solvent outweighs the former.