The Gas- and Condensed-Phase Efficacy of Functionalized Phosphorus Flame Retardants for Cotton Fabric: Phenyl vs. Phenoxy Groups

This study explores how functionalized aromatic P-FRs, specifically phenyl- and phenoxy-based phosphoric acid derivatives, influence the flame retardancy of cotton textiles. By systematically investigating derivatives with varying degrees of phenyl, phenoxy, and acidic hydroxyl terminations, alongside ortho-phosphoric acid as a reference, this work aimed to elucidate the role of aromaticity and functional group composition on both gas- and condensed-phase flame retardant efficacy. Cotton fabrics were treated with comparable phosphorus loadings (~3 g/m²), quantified using inductively coupled plasma optical emission spectroscopy (ICP-OES), to evaluate the gas- and condensed-phase efficacy of the flame retardants. Notably, derivatives with a higher number of acidic hydroxyl terminations exhibited the best flame retardant performance, enhancing char formation through dehydration and condensation reactions during combustion. Thermal analysis (TGA) and microscale combustion calorimetry (MCC) confirmed that phenoxy systems catalyze cotton decomposition more effectively, promoting dehydration through the hydrolysis of phenoxy groups. Furthermore, IR analysis of evolved gases revealed a significant reduction in volatile emissions for phenoxy systems, while this was not observed for phenyl derivatives. These findings underscore the importance of robust condensed-phase mechanisms for achieving effective flame retardancy in cotton textiles.