Enhancing flame retardancy of partial biopolyol-based polyurethane foams using phosphorylated lignin as an additive

Abstract

We report the development of polyurethane (PU) foams incorporating biomass-derived materials to enhance both performance and sustainability. Lignin and castor oil were partially substituted for conventional polyol at 10 wt % and 40 wt%, respectively. Additionally, lignin was chemically modified into phosphorylated lignin (PL) via a reaction with P2O5 under reflux conditions, serving as a flame retardant additive. Inductively Coupled Plasma (ICP) analysis confirmed a phosphorus content of 2.9 wt% in PL, while Fourier Transform Infrared (FTIR) spectroscopy and Phosphorus-31 Nuclear Magnetic Resonance (31P NMR) verified the successful bonding of phosphorus groups to lignin's hydroxyl groups. PL was then incorporated into partial biopolyol-based PU foams at concentrations of 5, 10, 15, and 20 wt%, alongside a comparative study using ammonium polyphosphate as a commercial flame retardant. The inclusion of PL in conventional PU foam formulations resulted in a reduction in firmness (from 326 kPa to 260 kPa), whereas its incorporation into lignin-based PU foams increased firmness (from 147 kPa up to 592 kPa) and in castor oil-based PU foams (from 207 kPa up to 341 kPa). Along with the observed increase in firmness of partial biopolyol-based PU foams, their flame retardancy improved by up to 42-65 %, though this enhancement was not directly proportional to the additive concentration. While ammonium polyphosphate provided higher overall firmness and thermal stability, PL exhibited comparable or even superior foam properties in improving both mechanical and thermal properties. These findings highlight the potential of lignin and castor oil as sustainable biopolyols for PU foam production, demonstrating the dual role of phosphorylated lignin as a flame retardant and mechanical enhancer.