Flame-Retardant Polyamide 6 Enabled by a Reactive α-Amino-ε-caprolactam-Based Comonomer and In Situ Melamine Cyanurate

To enhance the flame retardancy of polyamide 6 (PA 6) while retaining its processability and mechanical properties, we developed a synergistic strategy that combines reactive copolymerization with additive incorporation. A rationally designed, lactam-based phosphonamide comonomer (DCL) ensured compatibility and reactivity during the ring-opening polymerization of caprolactam and was copolymerized in the presence of melamine cyanurate (MCA). The resulting PA 6-DCL/MCA composites exhibited outstanding fire performance: PA 6 with 5 wt % DCL alone achieved an LOI of 29.5%, whereas the hybrid composite containing 3 wt % DCL and 2 wt % MCA reached a UL94 V-0 rating with nonflaming melt dripping. The flame-retardant mechanisms were systematically investigated using techniques including cone calorimetry, with a focus on the pyrolysis process and charring behavior. The flame suppression is attributed to a synergistic effect involving dominant gas-phase inhibition combined with condensed-phase action, the latter comprising heat dissipation through controlled melt flow and the formation of a slightly reinforced char barrier. This design provides a scalable, halogen-free route to high-performance PA 6, where the flame-retardant efficacy arises from the interplay of molecular structure and additive synergy.