A Core–Shell Elastic Flame Retardant with Superior Migration Resistance for Fire-Safe and Toughened Polyamide 66

A major challenge for halogen-free flame retardants is their tendency to migrate under high-temperature and high-humidity environments. For instance, the combination of aluminum diethylphosphinate (ADP) and melamine polyphosphate (MPP) used in polyamide 66 (PA66) easily migrated to the surface, leading to a white and frost-like appearance. To address this issue, a core-shell elastic flame retardant (SiR@FR) was prepared via a solution deposition method, wherein a polymethylsiloxane (SiR) layer was encapsulated on the surface of ADP and MPP. This shell not only improved the hydrophobicity of the FR but also the toughness of PA66. Experimental results demonstrated that PA66 with 9-SiR@FR (PA66-5) exhibited excellent migration resistance, with no visible surface whitening after 480 h of aging at 85 °C and 85% relative humidity. Meanwhile, PA66-5 displayed outstanding flame retardancy, achieving a UL-94 V-0 rating with an approximate 65% decrease in peak heat release rate compared with control PA66. Furthermore, SiR@FR enhanced the toughness of PA66 by alleviating stress concentration, resulting in a 21% increase in impact strength. This study presents a simple but reliable encapsulation strategy for fabricating flame-retardant PA66 composites that combine superior migration resistance and satisfactory mechanical properties, showing promising potential for demanding applications requiring long-term usability and stability.