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

With the wide application of epoxy resins in adhesives, electronic packaging materials, and aerospace fields, it is necessary to prepare high-performance flame-retardant epoxy resins to reduce the fire risk caused by their flammability. In this study, the rigid structure intermediate Schiff base (DMDA-SH) was synthesized by condensation reaction of syringaldehyde (SH) with O-Tolidine (DMDA). Then, DMDA-SH-DOPO, a novel P/N-structured biobased flame-retardant curing agent, was synthesized by addition reaction with 9,10-dihydro-9-oxaza-10-phosphame-10-oxide (DOPO) and was applied to the preparation of intrinsic flame-retardant epoxy resin. As expected, DMDA-SH-DOPO has good flame-retardant properties due to the synergistic action of N/P elements. Epoxy resin with only 2.5% DMDA-SH-DOPO (P = 0.16%) can pass the UL-94 V-0 test. Compared with DGEBA/DDM, DMDA-SH-DOPO-7.5's (P = 0.49%) peak heat release rate was reduced by 48.4% and the limiting oxygen index (LOI) reached 27%, making it a flame-retardant material. From the point of view of carbonaceous residue performance, the expansion height of carbon residue after DMDA-SH-DOPO-7.5 combustion is significantly increased, and the amount of carbon residue at 800°C is increased by 36.4%. In addition, appropriate DMDA-SH-DOPO can effectively improve the bending property of epoxy resin. This study provides a new idea for preparing renewable high-performance intrinsic flame-retardant epoxy resin.