Effect of compound system of bifunctional flame-retardant based on DOPS/phosphazene structure and ammonium polyphosphate on flame retardancy of epoxy resin

In order to obtain a flame-retardant epoxy resin (EP) composite with good flame retardancy, ammonium polyphosphate (APP) was applied to EP in combination with hexa-(4-(9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-sulfide)-hydroxymethylphenoxy)cyclotriphosphazene (HAP-DOPS). The experimental results showed that the flame retardancy of EP composite can be improved effectively when HAP-DOPS was compounded with APP and added to EP. Under the premise condition of the phosphorus content in the system was 1.6% and m _HAP-DOPS: m _APP = 1.5:1, the LOI value reached 30.0% and the vertical burning test reached UL94 V-0 rating, at which point the flame retardancy of the compound system was optimal. Compared with HAP-DOPS/EP, the thermal stability of HAP-DOPS/APP/EP was improved, and the char residues were increased; the total heat release (THR) was decreased by 40.66%, and the total smoke release (TSR) was decreased by 34.13%. And the flexural strength and tensile strength were increased by 32.52% and 14.46%, respectively. The study of flame-retardant mechanism showed that the addition of HAP-DOPS/APP to EP can promote the formation of phosphorus-rich char residues, which mainly played a flame-retardant role in the condensed phase and in the gaseous phase by releasing noncombustible NH₃.