Quantitative Evaluation of Surface Whitening Mitigation via Thermal Annealing in Carbon Fiber/Polyphenylene Sulphide Thermoplastic Composite Laminates

ABSTRACT Surface whitening is a manufacturing‐induced surface defect in thermoplastic composites, resulting from non‐uniform shrinkage and near‐surface resin instability during cooling. This defect can compromise subsequent coating, bonding, and inspection processes. This study examines thermal annealing as a post‐processing strategy for mitigating surface whitening in carbon fiber/polyphenylene sulphide (CF/PPS) laminates without modifying the existing forming process. Laminates were consolidated at 300°C under 10 bar, followed by annealing at 280°C under a contact pressure of 1 bar for 30, 60, and 120 min. Morphological analysis indicated that surface whitening is associated with localized resin protrusion, fiber exposure, and surface layer discontinuity, all of which increase incident light scattering. Thermal annealing promoted near‐surface resin reflow, reducing the whitened area fraction by 71.8%–80.5% across all conditions. Surface roughness parameters Sa and Sz decreased significantly, whereas the crystallinity and bulk thickness changed only marginally by up to approximately 1.5% and 2.6%, respectively. Thus, whitening mitigation is primarily driven by surface morphological recovery rather than by bulk structural reconfiguration. More broadly, these results demonstrate that controlled post‐process annealing can decouple surface‐quality improvement from primary forming‐condition modification, providing a practical route for improving the manufacturability, inspectability, and downstream process compatibility of semi‐crystalline thermoplastic composite laminates.