A novel organic-inorganic flame retardant of ammonium polyphosphate chemically coated by Schiff base-containing branched polysiloxane for polyamide 6

A novel organic-inorganic flame retardant (SP-APP) was first designed by coating ammonium polyphosphate (APP) with Schiff base-containing branched polysiloxane via ion exchange reaction and dehydration-condensation. High water resistance and charring capacity of the SP-APP greatly overcame the poor water resistance of APP and endowed polyamide 6 (PA6) with excellent flame retardancy and anti-dripping. The prepared PA6/SP-APP composite with 12 wt% of SP-APP reached the highest limited oxygen index (LOI) of 32.5% and passed V-0 rating of UL-94 test with non-dripping, while PA6 and PA6/APP with 12 wt% of APP failed to pass the UL-94 test. A sharp reduction of 50.9% in peak heat release rate (PHRR) and 29.4% in total heat release (THR) was found for PA6/SP-APP in contrast with PA6. The working mechanism of SP-APP in enhancing the flame retardancy of PA6 was also explored intensively and the synergistic effects of catalytic degradation of APP, self-crosslinking behavior of Schiff base structure and thermo-oxidative degradation of branched polysiloxane contributed to the improvement of flame retardancy of PA6/SP-APP.