Multifunctional Flame‐Retardant Polycaprolactone Interlayers for Carbon Fiber‐Reinforced Epoxy Resin Composites: Thermal Degradation, Fire Behavior, and Mechanical Performance

Reducing flammability and controlling damage processes are critical challenges in carbon fiber‐reinforced epoxy matrix polymer composites. The aim of the present study was to develop flame‐retardant polycaprolactone (PCL) filaments and to apply them as additively manufactured interlayers in laminated composite systems. The research builds on a previously validated PCL‐based solution that enables the control and localisation of mechanical damage. The novelty of the work lies in the multifunctionalisation of the interlayer, achieved by incorporating flame‐retardant additives into the PCL, allowing the layer to simultaneously fulfill mechanical stabilization and fire‐protection functions. The results demonstrate that PCL filaments modified with appropriately selected additives are compatible with additive manufacturing processes and, when applied as interlayers, favorably influence the fire response of composite laminates. The developed multifunctional layer may contribute to the advancement of intelligent, multifunctional polymer composite systems, as the introduction of a single interlayer enables the mitigation of different damage mechanisms.