Synthesis and characterization of a novel Schiff base and its enhancement on flame retardancy of epoxy resins

A novel Schiff base, namely N-1,N-2-bis{3,4-bis(5,5-dimethyl-2-oxide-1,3,2-dioxaphosphinan-2-yl)oxy]benzylidene}ethane-1,2-diamine (PN), was synthesized by the condensation of 3,4-bis[(5,5-dimethyl-2-oxido-1,3,2-dioxaphosphinan-2-yl)oxy] benzaldehyde (PCHO) and ethylenediamine. Its chemical structure was characterized by H-1, C-13, P-31 NMR, mass spectra and elemental analysis. PN was used as an additive flame retardant to improve the flame retardancy of diglycidyl ether of bisphenol A (DGEBA) cured by 4,4 '-diaminodiphenylsulfone (DDS) with different phosphorus mass fraction. The incorporation of PN promoted DGEBA/DDS to yield more char residues under both nitrogen and air atmosphere, and exhibit good flame retardancy. With 1.5 wt% phosphorus mass fraction, PN-1.5/DGEBA/DDS could reach the highest flame retardant level of V-0 rating for UL-94 vertical burning test, obtain a limiting oxygen index (LOI) value of 32.4%, and keep relatively good mechanical properties by 81.52% of flexural strength and 90.40% of Izod impact strength compared with DGEBA/DDS. The flame-retardant mechanism researches revealed that PN presented a comprehensive mechanism of condensed phase, gas phase and phosphorus-nitrogen synergistic effect in DGEBA/DDS system.