Modification of dry-spun Suplon polyimide fibers by mixed-acid oxidation and their effects on the properties of polypropylene-resin-based composites

In this study, the effects of mixed-acid oxidation on the contents of surface elements, morphology, fiber fineness, mechanical properties, mass change rate, chemical structure, and microaggregate structure of dry-spun Suplon polyimide (PI) fibers were systematically investigated with wet chemical treatment with HNO3/H2SO4. Experiments investigating both the improvement in the O/C ratio of the fiber surface elements and the changes in other performance features were conducted through the functional modification of the fibers. Meanwhile, the causes of specific changes in the mechanical properties of the oxidized PI-fiber-reinforced polypropylene-resin-based composites were studied and are discussed. The results of this study demonstrate that the treatment of the fibers with HNO3/H2SO4 mixed-acid oxidation resulted in significant changes in the properties of the fibers; these changes included an uneven surface, increased specific surface area and surface roughness, a locally etched surface, increased surface energy and O/C ratio, an enhanced wettability, an increased fiber fineness, reduced mechanical properties, and a mass gain in the fibers. Although the chemical structures of the fibers treated by oxidized HNO3/H2SO4 were not significantly changed compared to those of the untreated fibers, the microscopic aggregation of the treated fibers changed to some degree, and the ratio of the amorphous regions significantly increased. Taken together, the functional modification of the PI fiber surface was achieved efficiently through the use of a suitable HNO3/H2SO4 oxidation process and with other performance features of the fibers taken into account. This was favorable for the enhancement of the interfacial properties of the polypropylene fibers and the matrix resins, and thus, the modification improved the mechanical properties of the composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44932.