Preparation and characterization of cellulose phosphite ester with high flame retardancy via transesterification in BmimCl/DMSO system

The functionalization of cellulose via molecular structure design has garnered considerable attention. In the present work, cellulose phosphite ester with high flame retardancy was synthesized via transesterification in a co-solvent system of 1-butyl-3-methylimidazolium chloride (BmimCl) and dimethyl sulfoxide (DMSO), where dimethyl phosphite (DMP) served as the acylation reagent and 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) acted as the alkaline catalyst. The effect of the degree of substitution (DS) with phosphite group on the flame retardancy of the cellulose was investigated. The finding indicated that increased DS of cellulose phosphite ester led to enhanced flame retardancy of cellulose. Specifically, the prepared cellulose phosphite ester with DS of 0.35 exhibited an increase of 33.17% in char residues compared with microcrystalline cellulose (MCC). The heat release capacity (HRC) of cellulose phosphite ester was 51.6 J·g−1·K−1, which was 244.7 J·g−1·K−1 lower than that of MCC. Based on FT-IR, 13C and 31P NMR analysis, the characteristic peaks related to PO, P-O-C, P-H bond and phosphite group were present in the cellulose phosphite ester, confirming the successful transesterification of cellulose. Furthermore, SEM and Raman analyses further revealed that higher DS resulted in increased graphitization of char residues, which might be responsible for the improved flame retardancy of cellulose phosphite ester. Overall, this work introduced an eco-friendly and efficient approach to improving the flame retardancy of cellulose, potentially advancing the high-value utilization of cellulose-based products.