Epoxy resin modified with chitosan derivatives and DOPO: Improved flame retardancy, mechanical properties and transparency

Improving the flame-retardant efficiency while maintaining the good properties of epoxy resin has become increasing important for industry applications. In this work, a fast char-forming flame retardant (nitro-containing chitosan-derivate, named CSN) derived from a one-pot reaction between chitosan and p-nitrobenzaldehyde was synthesized. When the total addition amount of CSN and 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) in epoxy resin (EP) is only 5wt%, the cured EP passed the UL-94 V-0 rating and the limit oxygen index (LOI) reached to 34.0%. The total heat release (THR) and total smoke production (TSP) of the modified EP decreased by 23.5% and 77.2%, correspondingly, and the char-forming ability was significantly improved when compared with pure EP. Meanwhile, the CSN and DOPO modified EP maintained comparable tensile strength, flexural strength and transparency with pure EP, which is closely related to the abundant hydroxyl and amine groups as well as rigid structure in CSN and DOPO. Considering that the facile preparation method and small addition amount of the flame retardant, the present work provides a convenient solution for the preparation of modified epoxy resins with good flame retardancy, heat stability, transparency and comparable mechanical properties with pure EP, which is expected to be widely used in industries.