Organic-inorganic hybrid epoxy vitrimers with excellent thermal-mechanical stability based on carboxylic-acid type carbon dots as curing agent: Flame retardancy and reprocessing

Despite being the earliest and most extensively studied class of vitrimers, transesterification epoxy (EP) vitrimers still face some large limitations in thermomechanical stability and flammability. Unlike the preparation of functional composite materials using ordinary curing agent compounds and nano fillers, this paper proposed a new strategy of using carbon dots to design nano-curing agents and dynamically crosslink active sites for constructing epoxy vitrimers with excellent thermomechanical stability and flame retardancy. Herein carboxylic acid type carbon dots (C-CDs) was synthesized via glutamate pyrolysis, subsequently grafting with organophosphorus compounds via the Kabachnik-Fields reaction to achieve the phosphorus doping (P-CDs). It is interesting that the synthesized P-CDs played a quadruple role as flame retardants, curing agents, dynamically exchangeable bonds, and nano-reinforcement effect. The results showed that this new kind of organic-inorganic hybrid flame-retardant EP/CDs vitrimers was firstly reported, which exhibited good thermomechanical stability, flame retardancy and fast stress relaxation time. When the molar ratio of P-CDs to epoxy was about 1:1, the peak heat release rate (PHRR) and CO2 production (CO2P) of the EP/P-CDs could be reduced by 53.4 %, and 65.1 %. The limiting oxygen index (LOI) was 31.3 % and the UL-94 grade reached V-0 rating, which benefitted from the dual-phase flame-retardant mechanisms. Furthermore, the stress relaxation time of EP/P-CDs was significantly shortened to be 90 s, which was important for the reprocessing of epoxy thermosets. Notably, EP/P-CDs maintained up to 98 % of its thermo-mechanical properties even after twice reprocessing. This kind of organic-inorganic hybrid flame-retardant EP/CDs vitrimers will have imposing potential applications in many fields.