Methylcellulose-based composite hydrogels with high water retention and strong flame retardancy

In this study, a green flame-retardant thermo-sensitive hydrogel was developed, comprising methyl cellulose (MC) as the carrier, a phytic acid (PA)-urea (U) composite (PAU) as the flame retardant and calcium cation (Ca 2+ ) as the water absorbent. When applied to fabrics, the hydrogel undergoes a phase transition upon exposure to high temperatures, forming a robust barrier that significantly enhances flame retardancy. The samples were characterized using Fourier-transform infrared spectroscopy (FTIR), rheology, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The water absorption and retention properties of the MC/PAU/Ca coating were also evaluated, demonstrating superior absorbency, water retention, and thermal stability, which collectively contribute to improved flame retardancy. Compared with the untreated fabric, the MC/PAU/Ca flame-retardant coated sample exhibited a limiting oxygen index (LOI) value of 44.8 ± 1.5 %, and vertical flame testing (VFT) confirmed a UL-94 V-0 rating. The flame-retardant mechanism of coating in the gas phase and the condensed phase were effectively investigated by the FTIR of char residue and Thermogravimetric analysis-infrared spectrometry (TG-IR) analysis. During combustion, the coating generated a dense char layer enriched with phosphorus (P), nitrogen (N), and calcium (Ca), along with the non-flammable gases, which synergistically acted as an effective flame retardant mechanism. Overall, the MC/PAU/Ca hydrogel represents a promising green flame-retardant coating for flammable materials. • A new type of hydrogel composed of methylcellulose, PAU and Ca 2+ was prepared. • The hydrogel showed temperature sensitivity and high water absorption. • Hydrogel-coated acrylic fabric was nearly undamaged when exposed to fire for 20s. • Fire-retardant mechanism is due to a multiple synergistic pathway.