Exploring the impact of siloxane networks on the thermal behavior of P/N-enriched flame-retardant finishes for cotton fabric

Our aim was to decipher potential synergy between phosphorus‑nitrogen (PN) and silicon (Si) in flame-retardant (FR) coatings for cotton textiles using a novel TRIAMO-based phosphoramidate-FR (TD) as a benchmark. For that we chose a comparative approach, employing a silica-free diethylenetriamine based derivative (DD), a dual-layer coating of tetraethyl orthosilicate (TEOS) and DD (DD/T). Standardized flame tests revealed that TD exhibited reduced performance at low add-on compared to DD, while DD/T consistently showed improved performance across loadings. TG-IR studies demonstrated that phosphoramidate species degrade via the release of triethyl phosphate (TEP) into the gas phase while forming PN networks on the substrate, independent of silica presence. However, functionalized silane networks accelerate the condensed phase mechanism of TEP by limiting gas diffusion. This was evidenced by XPS as TD yields a dense network of aliphatic PNO species such as phosphoramides and phosphor oxynitrides, while DD predominantly forms phosphazenes. These findings align with isoconversional kinetic studies showing higher activation energies for TD-treated cotton, attributed to diffusion limitations induced by the sol-gel network. This barrier effect also inhibits cotton pyrolysis, and enhances the structural integrity of char, as evidenced by SEM, DSC, and DTG data. While TEOS in DD/T alone does not alter the FR mechanism, it synergistically enhances char formation without impeding dehydration activity. In contrast, functionalized silanes demonstrate synergy only at critical add-ons; at lower loadings, the barrier effect disrupts the interaction between the FR moiety and the substrate. These findings suggest that the passivation and barrier effects of silica networks, though beneficial, may be overestimated in functionalized silane coatings for reactive substrates like cotton, emphasizing the need for increased loading levels to achieve desired FR performance. The same conclusion was found for functionalized phosphonate (DPTS) and amine-based silanes (TRIAMO), demonstrating that our findings are applicable for systems beyond phosphoramidate.