Char to the Rescue: Processing and Transfer of Flame‐Retardant Epoxy Resins, Adjusting the Fire Behavior and Post‐Fire Structural Integrity of Glass Fiber Composites

ABSTRACT Epoxy (EP) glass fiber reinforced composites (GFRCs) are extensively used in structural applications due to their excellent thermal and mechanical properties, but their inherent flammability limits fire‐safe deployment. While numerous studies examine the flame retardancy of resins or individual composite systems, comprehensive studies evaluating simultaneous improvements in flame retardancy and post‐fire mechanical integrity, specifically through prepreg processing, remain challenging. This study investigates the effectiveness and transferability of phosphorus‐based flame retardant (FR) systems to diglycidyl ether of bisphenol A (DGEBA) and EP novolac resin matrices and their corresponding bidirectional glass fiber composites via prepregs. The FRs are chosen based on varying modes of action: ammonium polyphosphate with inorganic silicate (APP/InSi) primarily acts in the condensed phase (CP), and aluminum diethyl phosphinate with zinc hydroxystannate (AldietPO2/ZHS) is known to demonstrate both gas‐phase (GP) and CP activity. Fire residues are tailored to compensate for structural defects from fire exposure. EP novolac, with higher aromaticity and cross‐linking, possesses better inherent flame resistance compared to DGEBA. The novolac composites containing AldietPO2/ZHS simultaneously showed the highest retention of flexural properties after fire exposure and the best fire safety index. The experimental values of the post‐fire flexural properties in the composites calibrated damage parameters in two theoretical models.