Formation of skin-core in carbon fibre processing: A defect or an effect?

The influence of stabilized fibre structure and skin-core formation induced by rapid thermal stabilization of polyacrylonitrile (PAN) on the tensile properties of carbon fibres was investigated. Three sets of samples were prepared by stabilizing PAN fibres under three temperature profiles using a continuous carbon fibre processing line. Initially, the chemical structure and density variations in stabilized fibres were examined with respect to process conditions using Fourier Transform Infrared Spectroscopy (FTIR) and density column methods. Interestingly, while the cyclization and dehydrogenation indices are similar for all the stabilized fibres irrespective of temperature profiles used, the densities of these fibres varied from 1.34 to 1.366 g/cc. Micro-Raman studies showed the existence of structural heterogeneity in the fibres from low temperature
(LT) carbonization (I(D)/I(G) ratio of core was ~5.6% higher than the skin) that eventually reduced with high temperature (HT) carbonization because of uniform sp3 to sp2 hybridization of carbons. However, modulus mapping revealed heterogeneous storage modulus distribution in the HT carbon fibre cross-section from Trial-2 (storage modulus of core was ~23 GPa less than the skin). Interestingly, this heterogeneity did not show a significant effect on the bulk properties of carbon fibres suggesting skin-core formation is an effect rather than a defect.