Bio-based phytic acid and urea interfacial layer by layer assembly for flame-retardant cotton

In this work, LBL technology is applied to prepare a bio-based flame- retardant coating (U@PA-Na) by a sequential assembly of positively charged urea (U) and negatively charged sodium phytate (PA-Na) solution in water medium. The U@PA-Na system possess the characters of safety, non-toxic, and high efficiency in flame retardant. The flame retardancy of U@PA-Na is evaluated by the cotton coated with U@PA-Na coatings (COT/U@PA-Na) before and after washing treatment. The limiting oxygen index (LOI), vertical flame test (UL-94), and cone calorimeter test (CCT) composites are measured to evaluate the flame retardancy of COT/U@PA-Na. The LOI value of the COT/U@PA-Na3 composite after washing treatment can still reach 34.6% and pass UL-94 B1 grade, accompanied by damage length less than 150 mm and burning time less than 5s. It shows that U@PA-Na has excellent flame retardancy and water resistance. Cone calorimeter (CCT) measurements show that the peak heat release rate (PHRR) and total heat release (THR) of COT/U@PA-Na3 decreas by about 61% and 74% compared with untreated fabrics. The thermal decomposition time of the treated cotton fabric is significantly delayed, and the peak decomposition temperature (T1max) is reduced by about 62°C, while the average residue is improved to about 21% at 650 °C. The image of the cone calorimetric burnt carbon slag shows that the carbon layer of cotton fabric treated by U@PA-Na is very dense, indicating that the synergistic effect of PA and U can promote the formation of a high-quality carbon layer which plays a "barrier effect" to protect the substrate and delay the combustion process. The flammability and forced combustion tests show that the bio-based U@PA-Na flame-retardant system with good washing resistance can significantly improve the flame retardancy of cotton fabric, greatly reducing the fire risk.