A colorimetric framework for PAN fiber stabilization: defining the coordinate inversion point as an optical marker of oxidation saturation

Abstract

Thermal stabilization of PAN fibers is essential for producing carbon fibers, but tracking the process is challenging due to the use of destructive testing methods and the unreliable visual interpretation of color changes. This study presents a systematic colorimetric methodology to quantitatively link the optical response of PAN fibers during air stabilization to their chemical, thermal, and combustion transformations. A key contribution of this work is the definition of the Coordinate Inversion Point (CIP), a distinct optical marker characterized by the simultaneous inversion of multiple colorimetric trends, including lightness (L*), chromaticity coordinates (a* and b*), Chroma (C*), total color difference (triangle E*00), and RGB convergence. The CIP corresponds to the saturation of surface-driven primary oxidation, clearly distinguishing optical saturation from complete chemical stabilization in the bulk. These results highlight a significant skin-core effect, showing that visually identical black fibers at the CIP stage exhibit only intermediate internal stabilization (BRI approximate to 26%, RCI-1 approximate to 53%), underscoring the inherent limitations of subjective visual inspection in assessing advanced stabilization states. Regression analyses reveal a clear sensitivity hierarchy (B > G > R), with the Blue (B) component of RGB channels exhibiting strong quadratic correlations with the Burning Resistance Index (BRI, R-2 = 0.97), the IR-conversion index (RCI-2, R-2 = 0.96), and char yield (R-2 = 0.86). Overall, the CIP framework advances colorimetry as a quantitative tool for nondestructive assessment and improved control of PAN fiber stabilization. [GRAPHICS]