Mechanochemistry for the Synthesis of a Sustainable Phosphorus/Potassium Tannic Acid Flame-Retardant Additive and Its Application in Polypropylene

Tannic acid (TA) is an abundant biobased aromatic compound that can form char under thermal degradation and fire conditions. In the present study, TA was chemically modified by grafting potassium phosphate groups using a two-step process, requiring neither heating nor organic solvents. Initially, TA was functionalized through ball-milling mechanochemistry in the presence of P₂O₅ to graft phosphoric acid, which was later converted to potassium phosphate salt by simple precipitation in water in the presence of KOH. The reaction yield was 87%. The resulting product, namely, TA-P-K, was used as a flame retardant (FR) in polypropylene (PP). At 30 wt %, TA-P-K was dispersed in PP by melt processing, resulting in a significant reduction in the peak heat release rate of 78% measured by the mass loss cone calorimeter test. Additionally, an intumescent residue was formed during combustion, which protected the material. In terms of flame-retardancy performance, the polymer composite took a “Good” label based on Flame-Retardancy Index (FRI), which seems promising in view of FR being fully biobased.