A universal strategy toward flame retardant epoxy resin with ultra-tough and transparent properties

Flame-retardant epoxy composites have important application value in flexible optoelectronic devices, artificial intelligence and 5G fields. The flame retardancy of these composites, however, are often promoted at the expense of mechanical properties and transparency, particularly the toughness. In this work, we propose a simple strategy of introducing proper rigid-flexible groups into phosphorus-based flame retardant which realizes a dual performance improvement simultaneously in epoxy resin. The modified flame retardant epoxy resin has a limiting oxygen index (LOI) value of 32.3%, a high transmittance of approximately 92%, an excellent smoke and heat suppression properties. Remarkably, the addition of phosphorus-based flame retardant containing rigid-flexible groups (TOD) significantly enhances the mechanical properties of epoxy resin, especially the toughness. The impact strength is obviously increased from 6.8 kJ/m2 to 25.2 kJ/m2, with an increase of 271%, which mainly due to the flexible Si-O segment in TOD. This work provides a universal strategy for the design of flame retardant, ultra-tough and transparent epoxy resin, implying its enormous potential to promote the commercial application of epoxy resin.