Biobased Lignin as a Sustainable Flame Retardant Additive for Recycled Polyester Fabrics

Recycled poly-(ethylene terephthalate) (rPET) textiles present intrinsic challenges for flame-retardant finishing due to the hydrophobic and chemically inert surface of PET fibers, as well as their pronounced melt-dripping behavior during combustion. Here, we demonstrate that incorporating only 1% owf lignin into a phosphorus-nitrogen coating is sufficient to achieve effective flame retardancy (LOI of 37.5%, UL 94 V-0), while retaining self-extinguishing behavior after five laundering cycles. A sustainable halogen-free coating system was developed by combining a phosphorus-based flame retardant (aluminum diethylphosphinate) with biobased lignin and sericin within a cross-linked poly-(vinyl alcohol)/citric acid matrix. The incorporation of lignin promoted char formation, suppressed melt dripping, and facilitated rapid self-extinguishing behavior. Increasing lignin loading to 3-5% owf did not further improve UL 94 performance and instead reduced wash durability, indicating that low lignin loading is sufficient for effective performance. In addition to enhanced flame retardancy, the treated fabrics maintained mechanical integrity, exhibited hydrophobic surface characteristics, and showed good colorfastness to crocking.