An aromatic macromolecular polyimide (API) was synthesized and characterized, and used as a synergistic charring flame retardant in glass fiber reinforced polyamide 6 (GF/PA6). API and aluminum diethylphosphinate (ADP) exhibited better flame inhibition behavior and synergistic charring flame retardant behavior compared with ADP alone. The 5%API/7%ADP/GF/PA6 sample achieved the lower peak value of the heat release rate (pk-HRR) at 497 kW/m² and produced higher residue yields of 36.1 wt.%, verifying that API and ADP have an outstanding synergistic effect on the barrier effect. The API/ADP system facilitated the formation of a carbonaceous, phosphorus and aluminum-containing compact char layer with increased barrier effect. FTIR spectra of the residue and real-time TGA-FTIR analysis on the evolved gases from PA6 composites revealed that API interacted with ADP/PA6 and locked in more P–O–C and P–O–Ar content, which is the main mechanism for improving flame inhibition and charring ability. In addition, the API/ADP system improved the mechanical properties and corrosion resistance of GF/PA6 composites compared to ADP alone.

The synergistic charring, flame-retardant behavior of the macromolecular charring agents polyimide (PI) and melamine polyphosphate (MPP) were studied in glass fiber-reinforced polyamide 66 (PA66). This kind of synergistic charring effect is explained by the fact that PI performed better char-forming ability while working with phosphorus content. The research results showed that, compared with the incorporation of individual MPP, MPP/PI with an appropriate ratio exhibited better flame retardancy and better charring ability. A blend of 11.9%MPP/5.1%PI/PA66 possessed an increased LOI (limiting oxygen index) value of 33.9% and passed the UL94 V-0 rating, obtained a lower peak heat release rate value (pk-HRR), a lower total heat release (THR) value, a lower total smoke release (TSR) value, and a higher residue yield. The results verified the synergistic flame-retardant effect between MPP and PI in the PA66 composite. Melamine polyphosphate and PI jointly interacted with PA66 matrix and locked more carbonaceous compositions in residue and formed a more compact char layer, resulting in a reduced burning intensity and a reduction in the release of fuels. Therefore, the enhanced flame-retardant effect of the MPP/PI system is attributed to the higher charring ability and stronger barrier effect of the char layer in PA66 in the condensed phase.

In this work, the effect of a modified metal organic framework material on the fire resistance and mechanical properties of epoxy resin (EP) has been explored. The cobalt based metal organic framework (ZIF-67) was synthesized from an organic ligand with a Schiff base structure. Then DOPO@ZIF-67 was synthesized by modifying ZIF-67 with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and its effect on EP modification was explored. Compared with the pure EP, 4% DOPO@ZIF-67/EP passed the UL94 V-0 level and the ultimate oxygen index (LOI) reached 32.1%. The SEM pictures of carbon residue indicated that DOPO@ZIF-67 formed a more continuous and dense microstructure, which can enhance the thermal barrier and the physical barrier effect. The addition of DOPO@ZIF-67 also can effectively improve the stiffness and damping coefficient of EP composite material. The porous skeleton structure of DOPO@ZIF-67 can endow EP with rigidity and flame-retardant properties. Furthermore, the existence of DOPO made the combination of ZIF-67 with EP better. The results of this study suggest that DOPO@ZIF-67 may be a good additive for modification of the properties of epoxy thermosetting materials.