Synthesis of a novel chitosan Schiff base flame retardant for simultaneous improvement of fire safety, UV shielding and toughness of epoxy resins

Epoxy resin (EP) coating is characterized by high flammability, low resilience, and a pronounced susceptibility to aging. In this study, a novel chitosan-based Schiff base derivative flame retardant, STLCP, was synthesized using tert -butyl dimethyl chlorosilane, syringaldehyde, and DOPO as raw materials, with chitosan as the basic skeleton. The resulting EP/STLCP composites had excellent flame retardancy, UV shielding, and high toughness. The incorporation of only 1 wt % STLCP can effectively suppress the risk of combustion-induced melt dripping in EP. At a 6 wt % loading, the EP composite attained a UL-94 V-0 classification and an LOI of 32.3 %. In addition, the EP/STLCP-6 % composite demonstrated reductions of 46.3 %, 12.8 %, and 14.8 % in total heat release (THR), peak heat release rate (PHRR), and smoke production rate (SPR), respectively, when compared with pure EP. Flame retardant mechanistic analysis revealed that adding STLCP notably enhanced the char layer's graphitization degree, diluted combustible gas concentrations, and contributed synergistically to flame retardation in both the condensed and gas phases. In addition, STLCP exhibited the excellent UV aging resistance to EP, as the T UVA and T UVB transmittance values were reduced to below 3 %. Regarding mechanical properties, STLCP improved the tensile strength and impact toughness of EP by up to 43.3 % and 40.0 %, respectively, and improved the elongation at break by 24.6 %. Moreover, the EP/STLCP composite was shown to possess excellent thermal conductivity. This work is expected to provide a promising strategy developing green, eco-friendly, and multifunctional EP composites.