Flame-retardant, transparent, mechanically-strong and tough epoxy resin enabled by high-efficiency multifunctional boron-based polyphosphonamide

It is of great significance to develop high-performance epoxy resins (EPs) combining superior flame retardancy, smoke suppression, thermal oxidation stability, transparency and mechanical properties. However, current flame retardant design strategy usually realizes satisfied flame retardancy at the expense of other properties. Herein, a novel multifunctional high-efficiency boron-containing polyphosphonamide (PB) is synthesized, and its impacts on integrated properties of EP are studied thoroughly. As expected, the well-designed PB features high flame retardant efficiency due to the combination of phosphorus, boron and nitrogen elements, only 3 wt% of which increases the limiting oxygen index (LOI) and UL-94 classification of EP thermoset to 32.2% and V-0, with a ~32.2% decrease in the peak smoke production rate (PSPR). Compared with the unmodified EP sample, the tensile strength, elongation at break and impact strength of EP sample with 3 wt% PB are elevated by ~29.8%, ~37.7% and ~50.2%, respectively. Meanwhile, high glass transmission temperature and transparency are maintained. Hence, this work offers an integrated strategy to develop highly efficient multifunctional polyphosphonamides for the preparation of high-performance flame-retardant and smoke-suppressive EPs combining high mechanical strength and toughness, thermal resistance and transparency, which are expected to meet the increasingly rigorous industrial requirements.