Towards the Development of Innovative Bio-Based Flame-Retardant Systems Derived from Phosphorylated Condensed Tannins

In this study, tannins were investigated as sustainable flame retardants. The thermal and structural properties of tannic acid, chestnut, oak, and quebracho extracts were first characterized by Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), and Pyrolysis Combustion Flow Calorimetry (PCFC). Quebracho exhibited the most promising thermal performance, showing the lowest heat release and the highest char yield (≈38 wt.%). To further improve their efficiency, quebracho tannins were phosphorylated using phytic acid (QPHYA) or phosphoric acid (QPHOA). Phosphorus contents reached 13% and 15%, respectively, as confirmed by ICP and SEM–EDX analyses, while FTIR and NMR evidence the formation of new P–O bonds. Phosphorylation enhanced thermal stability, increasing char yield up to 52%. Flax rovings (TR, 500 tex) were treated with tannin formulations using an impregnation line. The treated rovings exhibited higher impregnation efficiency (Δ% ≈ 18) and phosphorus uptake (up to 3.7 wt.%). SEM–EDX analysis confirmed deeper penetration of the tannin-based agents within the rovings, while FTIR spectra revealed the appearance of phosphorus-related P–O and P=O absorption bands, suggesting phosphorus incorporation and treatment-induced interactions. TGA results demonstrated that phosphorylated rovings produced significantly higher char residues (33%–38%) compared to untreated flax (16%) and non-phosphorylated counterparts (≈21%). Overall, these findings highlight phosphorylated quebracho tannins as efficient and eco-friendly flame retardants, where the impregnation efficiency and phosphorus retention are strongly influenced by the fiber architecture.