Aqueous Kabachnik -Fields synthesis of HMF-functionalized chitosan for thermally-stable LBL coatings on cotton fabrics

• KF-modified chitosan shows increased thermal stability and char yield in air TGA. • Reaction temperature controls phosphorus incorporation and LBL cotton coating compatibility • The product produced at 80°C exhibits highest the strongest thermal screening performance, with delayed degradation and compact char. • Production at 60°C gives highest cotton mass gain in LBL coatings and strongest synergy (226.6%). • Synergy analysis shows KF-modified chitosan-Phytic Acid systems outperform unmodified LBL. This study explores the Kabachnik-Fields (KF) reaction between chitosan, diethyl phosphite (DEP) and biorefinery-derived 5-hydroxymethylfurfural (HMF) to produce chitosan-diethyl phosphite-HMF (CDH) derivatives for thermally-stable coatings. The effect of reaction temperature on the structure, phosphorus incorporation and thermal properties of the CDH-RT (room temperature), CDH-60 (60 °C), CDH-80 (80 °C) and CDH-100 (100 °C) products was investigated using FTIR, elemental analysis, ICPOES and thermogravimetric analysis (TGA) in air. KF modification increased chitosan's initial thermal stability (T₅: 65 to 84-104 °C), promoted early char formation through an intumescent mechanism and enhanced residue stability at 930 °C (max. 32.4 wt%). To assess LBL coating compatibility, CDH products were applied to cotton fabrics (3 bilayers with phytic acid). Mass gain depended on KF temperature, with cotton-CDH-60/PA showing optimal incorporation (34.4%). Synergy factors calculated from normalized TGA residues confirmed positive char-promoting effects for all CDH formulations (cotton-CDH-60/PA: 226.6%). These results establish HMF-chitosan aminophosphonates as a promising platform for sustainable, thermally-protective cotton fabric coatings.