A P/N synergistic flame retardant containing flexible nitrile group for enhancing the mechanical properties and fire safety of epoxy resins

To further enhance the fire safety of epoxy resins, a phosphorus–nitrogen synergistic flame retardant (DPVHD) with a flexible chain segment and a nitrile group was synthesized. In addition to the traditional flame retardant elements P and N, the flame retardancy of the epoxy composites is further enhanced by the cross-linking effect of the nitrile groups introduced to form an interpenetrating network with the epoxy matrix. The results showed that the sample with 10 wt% DPVHD-10/EP had a LOI of 32.3% and passed the UL-94 V-0 rating, with PHRR and THR reduced by 15.4% and 47%, respectively, compared to pure EP. It is worth noting that the introduction of flexible chain segments improves the flexural properties of the epoxy resin matrix. Compared with pure epoxy, the flexural properties of epoxy-cured compounds with DPVHD addition show a rising trend with the increase of flame-retardant additive. And the rise of DPVHD-10/EP reaches 33.6%, which greatly reduces the negative effect of flame-retardant additives on the mechanical properties of the epoxy resin matrix. This work therefore provides new ideas for exploiting the advantages of phosphorus-nitrogen synergy and nitrile-based cross-linking for flame retardancy in epoxy resins.