Pre-oxidation of cellulose controlled by the nitrogen-phosphorus compound catalyst to prepare fibers with ultra-high flame retardancy

Cellulose fibers are a class of green fibers that are widely used. But its flammability makes it a very high fire risk. According to inference, glycine and phosphoric acid may have the function of binding each other and capturing cellulose pyrolysis fragments. This is of great significance for the exploration of the hypothesis that the oxidation process of cellulose can be regulated by adding catalysts so that the oxidized fibers have high carbon yield and ultra-high flame retardancy. In this study, a compound catalyst was prepared with glycine and phosphoric acid in a particular ratio for the first time, and the chemical structure of lyocell fiber during pre-oxidation was adjusted with a specific step-by-step heating system to prepare flame-retardant fiber. The results of flammability testing showed that the limiting oxygen index (LOI) of the oxidized fibers unexpectedly jumped to 66.4%, more than three times higher than that of lyocell fibers with an LOI of only 18%. The UL-94 grade also directly reached the highest level (V-0). This means that the study produced fibers with excellent flame retardancy with a simple method, which to some extent verified the previous hypothesis. Furthermore, based on the abundant XPS and FT-IR data, a unique mechanism for the synergistic flame retardancy of N and P based on the phosphoramide structure is also proposed. This study provides a new method for broadening the application scope of cellulose-based products in the field of fire risk.