PARTIAL CARBONIZATION OF ARAMID FIBERS

Heat treatment of aramid fiber was conducted in the temperature range 300-710 degrees C nominally for 10 and 30 s in both static air and flowing nitrogen atmosphere. Crystallinity, crystal orientation, and crystallite size were determined using x-ray diffraction. Fibers with a skin-core structure were produced at intermediate temperatures, as revealed by scanning electron microscopy of fibers after partial dissolution of the fiber in 95-98% sulfuric acid. The skin, which forms in both nitrogen and air, is amorphous and brittle. It is insoluble in sulfuric acid, suggesting it is a cross-linked polymer. Formation of the skin may be facilitated by the removal of an aggressive chemical species that forms during heat treatment. The species may diffuse out of the outer layer of the fiber, allowing it to cross-link. The molecular weight of the dissolved core, analyzed using intrinsic viscosity, decreases with increasing heat treatment temperature. The tenacity, modulus, elongation-to-break, and toughness of fibers with a skin-core structure decrease with heat treatment and the fiber loses its fibrillar character. Mechanical property reductions are greater in air than nitrogen. X-ray data are also consistent with the notion that oxygen assists attack of crystals at high temperatures. Scanning electron microscopy shows that fibers have become skin-core composites with quite different mechanical properties between the two regions. A fiber failure mechanism is proposed. (C) 1994 John Wiley & Sons, Inc.