Efficient Hyperbranched Flame Retardant Derived from Quercetin for Use in Epoxy Resin with Well-Balanced Comprehensive Performance

Traditional flame retardants, often derived from petrochemical sources, pose significant environmental and health concerns due to their potential toxicity and persistence in the environment. In this study, a biobased hyperbranched polymer flame retardant named QB was synthesized using quercetin and phenylphosphoryl dichloride by a one-step method. The QB copolymer was characterized via Fourier transform infrared spectroscopy, thermogravimetric analysis, and gel permeation chromatography, revealing its high thermal stability and polymeric nature, with a weight-average molecular weight of 78 299 g/mol. QB was subsequently incorporated into bisphenol A-type epoxy resins using 4–4 diamino diphenylmethane as a curing agent to prepare the flame-retardant epoxy composite. With additions of only 1 wt % QB, EQB-1 achieved a UL 94 V-0 rating in the vertical burning test and an impressive limiting oxygen index (LOI) value of 28.2%. Moreover, the addition of the 3 wt % QB in EP resulted in a maximum reduction of 32.9% in the peak of heat release rate and a 37.4% reduction in the smoke produce rate, indicating its outstanding flame-retardant and smoke suppression properties, which are attributed to a mainly condensed-phase flame-retardant mechanism. Furthermore, the impact and flexural strength of the composite were maintained and a slight improvement was observed. The findings of this research are expected to contribute to the development of environmentally friendly flame-retardant epoxy systems that meet industry standards while promoting the use of renewable materials. This work supports sustainability by replacing petrochemical flame retardants with renewable quercetin-based materials, reducing toxicity and environmental impact.