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

•A bio-based, phosphorus-phosphorus, hyperbranched oligomer (DIT) was synthesized.•DIT enabled EP to achieve durable fire safety.•DIT significantly improved mechanical robustness and toughness of EP.•DIT well maintained high transparency and thermal resistance of EP.