Durable flame-retardant, strong and tough epoxy resins with well-preserved thermal and optical properties via introducing a bio-based, phosphorus-phosphorus, hyperbranched oligomer

The manufacture of transparent, durable fire-safe, strong yet tough epoxy resins (EPs) remains a great challenge. Herein, a novel hyperbranched, P-P synergetic, bio-based flame retardant (DIT) was synthesized, which was composed by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), bio-derived itaconic anhydride (ITA) and trihydroxymethylphosphine oxide (THPO) moieties. Adding 2 wt% DIT enabled the EP/2DIT sample to achieve a UL-94 V-0 rating and a limiting oxygen index (LOI) of 36.6%. The peak heat release rate and total smoke production of EP/4DIT sample with 4 wt% DIT were 23.8% and 19.9% lower than those of virgin EP sample. Notably, the EP/DIT sample featured durable flame retardancy because of the oligomeric structure of DIT. The high flame-retardant efficiency of DIT was mainly due to the synergism between DOPO and THPO. DIT simultaneously strengthened and toughened EP thermoset at a low addition (≤2 wt%), and 2 wt% of DIT increased the elongation at break, tensile strength, flexural strength and impact strength of EP/2DIT sample by 35.8%, 40.5%, 24.2% and 21.2%. Moreover, the glass transition temperature (Tg) of EP/2DIT sample was equal to that of EP sample, and the high transparent was maintained. Hence, this work offers an all-rounded design for the creation of durable flame-retardant EPs with superior mechanical properties, thermal resistance and transparency by introducing hyperbranched, P-P, bio-based flame retardants, which shows huge potential and broad prospect.