Self-adhesive cellulose nanofibers achieve ultra-high flame retardancy via deep eutectic solvent mediated dual P/S functionalization

In this study, a cooperative system involving ammonium dihydrogen phosphate (MAP) and a binary deep eutectic solvent (DES) was designed, achieving dual functionalization (phosphorylation and sulfonation) of straw fibers through an innovative combined pretreatment scheme. The incorporation of sulfonic acid and phosphate groups facilitated the nanofibrillation of straw fibers, markedly decreasing their diameter. When coupled with brief ultrasonic treatment, cellulose nanofibers were effectively prepared. The introduction of sulfonic acid and phosphate groups enhanced hydrogen bonding interactions between the cellulose nanofibers, enabling self-adhesive straw cellulose nanofiber laminates to be produced. These laminates exhibited excellent flame retardancy and mechanical properties. Due to their low energy consumption, simple process, and readily available raw materials, cellulose nanofiber laminates have broad application potential in industries such as construction, design, and automotive interiors. Characterization revealed that the tensile strength increased more than threefold, thermal stability was enhanced, the limiting oxygen index (LOI) reached 74.9 %, and the heat release rate dropped by 74.3 %, indicating significant self-extinguishing capability. These findings suggest that the MAP–based DES system shows great promise for fabricating highly flame-retardant materials. The schematic diagram illustrating the sulfonation and phosphorylation processes along with their flame retardant mechanisms was presented. Partially delignified straw (DS) was treated with the MAP–DES system to produce phosphorus–sulfur straw (PSS). Following a brief ultrasonic treatment, PSCNF were obtained. Films were fabricated via vacuum filtration, and subsequently, laminates were constructed through hot pressing and moisture equilibrium treatment. The presence of sulfonic acid groups and phosphate groups facilitated the formation of a dense carbon layer at high temperatures. This, in conjunction with its unique layered structure, effectively inhibited heat transfer. Consequently, the prepared laminates demonstrated excellent flame retardancy and self-extinguishing properties. • Dual functionalization of straw fibers was achieved using a DES-assisted pretreatment. • Phosphate and sulfonic acid groups enhanced CNF flame retardancy and hydrogen bonding capacity. • Self-bonding enabled dense CNF laminates to form without any external adhesive materials. • Laminated structure showed excellent thermal stability and self-extinguishing behavior in combustion. • Dual-group modification promoted nanofibrillation and reduced CNF production energy and time costs.