Interphases Developed from Fiber Sizings and Their Chemical-Structural Relationship to Composite Compressive Performance

The relationship between interphase microstructure produced from fiber surface chemistry and the resulting composite structural properties is not well understood. This relationship is, however, key to predicting composite performance and to the understanding of how to assemble the constituents properly for best use of the unique properties of the fibers. In this study, a significant difference in laminate compressive performance is observed for both static and dynamic loading. Two different fiber sizings, an unreacted bisphenol-A-based epoxy and the thermoplastic, polyvinylpyrrolidone (PVP), are identified as the source of these observations. The fiber, matrix and processing remained the same in both systems. However, morphological differences in the interphase region were observed. The resulting mechanical properties indicated that the PVP sizing improved static compressive strength by 51% and increased the fatigue life of notched cross ply laminates (cycled at R=−1) by at least two orders of magnitude over the bisphenol-A-sized composites. The mechanism by which PVP brought about these changes is unclear and requires further study. However, these results also indicate that the selection of interphase characteristics requires additional information about the fiber-matrix bond than merely strength.