Effect of acid soak treatment of the precursor on the structure and properties of polypropylene-based carbon fibers

Abstract

Polyolefin fibers have received wide attention as the precursor for low-cost carbon fiber, and cross-linking of linear polyolefin by strong acid has been proven to be an effective way to stabilize the precursor fiber prior to carbonization. However, the conventional temperature used in sulfonation process is too high and could result in severe decomposition of the linear chain of the polymer, which further deteriorated the mechanical properties of the as-prepared carbon fiber. Thus, reducing the duration of sulfonation process operated at high temperature may alleviate the degradation behavior, conferring the polyolefin-based carbon fibers with better performance. In this paper, polypropylene (PP) fibers were soaked in sulfuric acid at 50 °C–90 °C, before the conventional preparation protocol of PP-based carbon fiber. The structural evolution from PP to carbon fiber were characterized by Differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and Raman spectroscopy. The mechanical and conductive properties of carbonized fibers where further measured. The results showed that the soak in sulfuric acid resulted in swelling of PP fibers in diameter direction, as well as reducing the crystallinity of the fibers, both of which were physical transformation, and could be easily controlled by soak temperature. The subsequent sulfonation at conventional temperature was fastened for the soaked PP fibers, and the duration of sufficient sulfonation were almost reduced by half. The obtained carbon fibers exhibited better thermal conductivity and mechanical properties, which were enhanced by 25 % and 40 %, respectively, compared with that without soak pretreatment.