Phytic acid derivatized lignin as a thermally stable and flame retardant material†

Phosphorus-containing flame retardants have attracted attention due to their outstanding flame retardancy, enhanced thermal stability, and limited toxic smoke emission. Bio-based phosphorus-containing flame retardants could be excellent options to impart environmental benefits, renewability, and sustainability to these materials. Lignin is an underutilized but abundant and sustainable material that can be used to serve this purpose. In the present work, a lignin-derived flame retardant was produced following the facile solvent-free polycondensation reaction of kraft lignin (KL) and phytic acid (PHA) at a low temperature in an aqueous system. The optimized conditions for this reaction were 1/0.4 mol/mol KL/PHA, pH 11, 20 °C, and 20 min. By utilizing advanced NMR (H, P, and HSQC), XPS, and FTIR techniques, the covalent bonding of the phosphorus of PHA with the oxygen of aliphatic and aromatic hydroxyl groups of KL was confirmed. C–P–O and P–O–P bonds provided high decomposition temperature (T_max), high glass transition temperature (T_g), and char formation in the product. The presence of phosphorus atoms was observed on the combusted material by EDS mapping and EDX, illustrating the increase in the intensity of this element after combustion at 800 °C. The results of this work provided a new approach for preparing a fully bio-based flame-retardant with limited smoke density (i.e., a decrease from 34% for KL to 17.7% for modified KL) and a higher limiting oxygen index (i.e., an increase from 21.8% for KL to 26% for modified KL) following a green chemistry concept.