Synergistic Phosphorus–Nitrogen Intumescent Back-Coatings with Endothermic Additives for Enhanced Flame Retardancy of Natural Fiber Upholstery Fabrics

The increasing use of natural-fiber textiles in interior applications requires effective and environmentally friendly fire protection solutions, as ignition of household fabrics remains a major cause of residential fires. Upholstered furniture manufacturers are particularly interested in advanced intumescent coatings for upholstery fabrics. This study presents a new intumescent flame-retardant system applied to natural-fiber interior fabrics using a back-coating technique commonly used in the upholstery industry. The formulation was based on phosphorus-containing resins produced under license from the Institute of Natural Fibers and modified with endothermic additives such as aluminum hydroxide, expanded vermiculite, β-cyclodextrin, dicyandiamide, and nanosilica. Flame-retardant performance was evaluated using an author-developed textile flammability method, the UL-94 vertical test, and cone calorimetry. Back-coated wool and linen fabrics showed significantly improved flame resistance while maintaining good mechanical properties. The optimized NEXP/D formulation achieved the best performance, obtaining a V-0 classification in the UL-94 test. Cone calorimetry demonstrated reductions of up to 68% in peak heat release rate and 85% in MARHE, together with increased char residue and gas-suppressing effects, especially for wool fabrics. The study provides a halogen-free approach to flame retardancy and supports the development of sustainable green interior textiles.