Synthesis of a hyperbranched polyamide charring agent and its flame-retarding and toughening behavior in epoxy resin

A hyperbranched polyamide charring agent (HPCA) was synthesized by amidation of 1,2,3,4-butanetetracarboxylic acid and melamine, and its chemical structure was characterized. Such a charring agent, combined with ammonium polyphosphate (APP), was applied in epoxy resin. V-0 rating in the UL-94 vertical burning test and 32.3% of Limiting Oxygen Index were achieved with only 10% total flame retardant loading. A HPCA/APP ratio of 1/1 showed better flame retardant properties than the system with only HPCA or APP involved, indicating an apparent synergistic effect. The residual char at high temperature was analyzed by using FTIR and X-ray Photoelectron spectroscopy (XPS) to further explore the above synergistic mechanisms. Additionally, HPCA, as a co-curing agent, was grafted into the crosslinking net of epoxy resin due to the reactivity of carboxylic acids with epoxy group and hydroxyl group. The introduced hyperbranched polyamide structure effectively improved the toughness of original rigid crosslinking net of epoxy resin.