Partially bio-derived phosphazene-tannic acid microspheres as fire retardant additives for an epoxy tannic acid resin system

In this work, partially bio-derived colloidal microspheres are synthesised from tannic acid (TA) and hexachloro-cyclophosphazene (HCCP), then investigated for their effectiveness as a fire-retardant additive to a partially bioderived epoxy resin system. The epoxy resin system is based upon diglycidyl ether of bisphenol A (DGEBA) and tannic acid (TA), a bio-derived highly aromatic phenolic curative noted for its inherent fire retardancy. This study presents the impact of increasing the concentration of the phosphazene-TA microspheres (TAPZ) and varying the DGEBA-TA stoichiometry to determine the optimum formulation required to achieve the greatest improvement in fire retardancy. At 15 wt% TAPZ addition, the DGEBA-TA resin system easily achieves a V-0 rating according to the UL-94 vertical burn test, measures 29 % for the limiting oxygen value (LOI) and displays a 62 % reduction in peak heat release rate (PHRR) compared with the unmodified DGEBA-TA network. TAPZ addition however reduces the glass transition temperature (T g ) from 187 °C to 155 °C, flexural strength by up to 30 % and increases viscosity significantly. Varying the stoichiometry between DGEBA and TA shows that additional surface hydroxyl groups from the TAPZ microspheres accelerate the DGEBA-TA reaction through enhancing interfacial reaction between residual epoxide groups of the cured epoxy network. Overall, these results present a promising approach to the development of a highly fire retardant, high performance, and highly bio-derived epoxy network and composite.