Enhanced flame retardancy in epoxy resins by ZnAl-LDH/ammonium polyphosphate synergism: impact of alkali source variation

Abstract

The application of ammonium polyphosphate (APP) as an additive flame retardant to epoxy resin (EP) flame retardants is common; however, APP poorly disperses and easily agglomerates in EP. For solving this problem, in this work, three kinds of ZnAl-layered double hydroxides (ZnAl-LDH1, ZnAl-LDH2 and ZnAl-LDH3) were successfully prepared using sodium hydroxide, urea and triethanolamine as different alkali sources, respectively, strengthening the flame retardant performance of EP with APP. Results showed that compared with pure-EP, the heat release peak and the smoke production rate peak decreased by 58.2% and 44.2% after the addition of APP (5 wt%) and ZnAl-LDH3 (5 wt%), respectively, which was prepared with triethanolamine as the alkali source. The incorporation of ZnAl-LDH3 promoted the dispersion of APP and increased the relative content of carbon elements during this process, as evidenced by SEM and XPS analyses. The metal oxides were uniformly distributed on the surface of the carbon layer after combustion. The residual carbon content of ZnAl-LDH3 was smaller than that of ZnAl-LDH1-APP-EP and ZnAl-LDH2-APP-EP, as observed in the Raman pattern. The heat release profile of APP-EP was better than that of ZnAl-LDH3-APP-EP; however, the tensile property and elongation at break of ZnAl-LDH3-APP-EP were better than those of APP-EP. A possible flame retardancy mechanism was proposed, according to which the C element introduced during the synthesis of LDHs promoted the cross-linking of EP and APP to form a denser carbon layer in the composite. Based on the above-mentioned results, triethanolamine shows potential as a source of both alkali and carbon in the preparation of LDHs. ZnAl-LDH3-APP-EP with good flame retardancy and mechanical properties could be employed as an efficient flame retardant material.