Sustainable flame retardant design for biodegradable polybutylene succinate using tea residue-derived additives

To address pressing environmental concerns such as net-zero emissions and the circular economy, this study presents a novel, sustainable intumescent flame retardant (IFR) system derived from agricultural waste, tea residues (TRs). TRs were combined with ammonium polyphosphate (APP) as an acid source and melamine (MEL) as a gas source at a mass ratio of 1:3:1 to form a synergistic TR/APP/MEL (TRAM) flame retardant, which was incorporated into biomass-based polybutylene succinate (PBS) to produce flame-retardant biocomposites. As an effort to further enhance charring efficiency, zinc oxide (ZnO) was introduced as a catalyst, preparing a second IFR system, TR/APP/ZnO (TRAZO). Combustion testing showed that incorporating 30 wt% TRAM into PBS increased the limiting oxygen index (LOI) from 22 % (neat PBS) to 40 % and achieved a UL-94 V-0 rating with no dripping. Thermogravimetric analysis (TGA) confirmed improved thermal stability, with char residue increasing from 1.98 wt% to 14.39 wt%. Additionally, the TRAZO-based composite containing only 20 wt% IFR also achieved a UL-94 V-0 rating and an LOI of 30 %, indicating that introduciton of ZnO effectively eliminated dripping of PBS composite by the graphitized char formation during combustion, thereby enhancing the flame retardancy. Cone calorimetry (CCT) results further demonstrated that TRAM significantly reduced the heat release and smoke production. These results highlight the potential of TR-derived IFRs, including TRAM and TRAZO, as green, halogen-free, and efficient flame retardants for biodegradable polymers. This approach supports the development of sustainable flame-retardant materials aligned with circular economy principles.