Polyelectrolyte as highly efficient flame retardant to epoxy: Synthesis, characterization and mechanism

• A flame retarding polyelectrolyte complex was prepared facilely and eco-friendly. • 4 wt% PPA-PEI endowed EP composite with satisfied flame-retarding and mechanical properties. • The char structure changed with increasing PPA-PEI loading. • The multicellular char residue showed superior physical barrier and insulation properties. To balance the flame retardancy and mechanical properties of epoxy (EP) resin, a polyelectrolyte complex (PPA-PEI, P content of 9.27 %) was facilely prepared in aqueous solution by the neutralization reaction between polyethyleneimine (PEI) and phenyl phosphonic acid (PPA). Just 4 wt% PPA-PEI could endow the relating EP composite with satisfied flame retardancy (limited oxygen index (LOI) of 29.0 %, UL-94 V-0 rating). It also shows significant reduction in total heat release (THR), peak heat release (PHRR), total smoke production (TSP) of 52.5%, 52.3%, and 22.7% respectively. Analyzed from the char morphologies and chemical structures, it can be induced that the good flame retardancy is mainly attributed to the highly yielded char residue with well swelled and multicellular structures possessing good barrier and thermal insulation properties. Meanwhile, the dilution and free radical scavenging effects also work well in the gaseous phase. Before the PPA-PEI molecules start to collapse into cluster seriously to worsen the dispersability within EP matrix, EP/4wt%PPA-PEI can maintain similar tensile strength and glass transition temperature ( T g ), and display improved Yong's and storage modulus compared with neat EP. Accordingly, this presented work provides a low-cost and promising approach for preparing flame-retardant EP/PEC composites with excellent comprehensive performance.