Synergistic flame retardancy of cross-linked kraft lignin and black phosphorus on TPU: Mechanism and performance

A novel, environmentally friendly flame retardant (FB) composite was developed by integrating cross-linked kraft lignin (FSK) and black phosphorus (BP) for thermoplastic polyurethane (TPU) applications. FSK was prepared by C C Friedel-Crafts crosslinking between benzene rings and amino/silanization between hydroxyl groups of lignin. The FB, with an optimal FSK:BP ratio of 7:3 at 1 wt%, significantly improved the flame retardant properties of TPU. It achieved a limiting oxygen index (LOI) of 30.7%, a 91% increase in char residue, and UL-94 V-0 classification, surpassing the performance of pure TPU. Cone calorimeter tests revealed a 21.26% reduction in total heat release (THR) and a 37.06% reduction in peak heat release rate (PHRR). The flame retardancy mechanism involved BP and FSK's crosslinked structure catalyzing the formation of a stable graphitized char layer, while FSK, through its nitrogen and silicon components, released inert gases such as NH 3 /CO 2 , enhancing the stability of the char layer. The synergistic effect provided dual-phase flame retardancy (condensed-phase and gas-phase) at low additive levels, maintaining mechanical properties (tensile strength: 20.1 MPa). This approach, using biomass-derived lignin and low levels of BP, offers an environmentally friendly alternative to traditional flame retardants, making it a promising strategy for high-safety applications.