Improving fire retardancy and mechanical properties of polyurethane elastomer by acid hydrotropic lignin

Improving flame retardancy and mechanical strength of lignin-containing polyurethane is a great challenge. In this study, lignin with favorable reactivity and dispersity was extracted from poplar using acid hydrotrope p-TsOH in EtOH. The extracted acid hydrotrope lignin (AHL) was subsequently functionalized with nitrogen and phosphorus (FHL) and reacted with isocyanate to fabricate a fire-retardant polyurethane (FHL-PU). The resulting FHL-PU exhibited a five-fold increase in fracture toughness and remarkable reprocessability, attributable to the dual cross-linked network formed by dynamic hydrogen bonds and carbamate bonds between AHL and PU. Furthermore, the FHL can effectively prevent the release of heat and smoke through mechanisms like forming a char layer at elevated temperatures, generating non-combustible gases and sequestering free radicals. As a result, the FHL led to a reduction in the peak heat release rate and total heat release of PU from 946.8 kW/m² and 86.9 MJ/m² to 383.5 kW/m² and 28.4 MJ/m², respectively. This acid hydrotrope lignin modified polyurethane, holds tremendous potential for a wide range of practical applications.