Multi-DOPO-based derivative for enhancing flame retardancy and mechanical properties of epoxy resin

Achieving a balance between flame retardancy and overall performance in epoxy resin (EP) continues to be a significant challenge. In this study, a new multi-DOPO-based reactive flame retardant (DZH) was synthesized successfully using a simple one-pot method and employed as a co-curing agent with DDM. The results indicated that the inclusion of DZH decreased the activation energy of the curing reaction system. Furthermore, the presence of DZH, with its remarkable charring ability, enhanced the formation of char residues under high temperature conditions. Notably, the flame retardancy and mechanical properties of the EP/DZH system were significantly enhanced at certain additions. When added at only 5 wt% (phosphorus content: 0.525 wt%), the LOI value of the EP/DZH-5 increased from 24.4 % to 35.3 % for the EP matrix and achieved a V-0 rating in accordance with UL-94. Meanwhile, EP/DZH-5 exhibited a reduction of 44.1 % in pHRR and 24.3 % in SPR. The superior flame retardancy of the EP/DZH system was attributed to the dual flame-retardant mechanism exhibited in both the gaseous and condensed phases. In addition, the abundant presence of rigid phosphaphenanthrene groups and 7C-7C interactions between benzene rings led to an improvement in the tensile strength and tensile modulus, which rose to 91.0 MPa and 1.76 GPa, respectively, from 79.5 MPa and 1.26 GPa of EP in the case of EP/DZH-3. The utilization of this flame retardant, based on multi-DOPO, offers a promising approach for the development of high-performance epoxy resins.