Surface modification of carbon dots by boron and phosphorus to construct flame-retardant epoxy resin with high mechanical and low dielectric properties

The effective combination of flame retardant element and carbon dots for the development of epoxy composites with high mechanical property and low dielectric constant in the field of flame retardant material is still a challenge. Herein, the boron and phosphorus dual-element doped carbon dots (BP&NCDs) was synthesized using a simple Kabachnik-Fields reaction, and then incorporated into epoxy resin in different amount to prepare a series of EP/BP&NCDs composites. The optical, thermal stability, mechanical, dielectric and flame-retardant properties were tested by many techniques such as UV–vis spectroscopy, FL, TEM, TG, UL-94 and LOI e.g. It showed that the EP/BP&NCDs composites had favorable transparency, hydrophobicity, mechanical and dielectric properties, while maintaining the photo-luminescent properties. It was worth noting that the EP/3 %-BP&NCDs composite with only 3 % BP&NCDs addition exhibited significant flame-retardant properties with a UL-94 V-0 rating and a limiting oxygen index (LOI) of 31.3 %. Meanwhile, it led to obvious reductions of 34.9 %, 26.6 %, and 15.6 % in peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP), respectively. Furthermore, compared with pure EP, the EP/3 %-BP&NCDs composites enhanced the bending strength and impact strength by 32.3 % and 30.3 %, but reduced the dielectric constant and dielectric loss by about 40.0 % and 50.0 %, respectively. When EP/BP&NCDs composites were burned, a continuous and dense carbon layer was formed due to the synergistic action of phosphorus and boron in CDs. Overall the carbon dots (CDs) after surface modification could be applied in epoxy resin system for the achievement of multiple functional EP composites with good flame retardancy, mechanical and dielectric property at low addition, which is helpful to provide a universal strategy in this field.