Phosphorylated cellulose propionate derivatives as thermoplastic flame resistant/retardant materials: influence of regioselective phosphorylation on their thermal degradation behaviour

Cellulose ester derivatives having phosphoryl side-chains were synthesized by phosphorylation of two types of cellulose propionate (CP); the difference between the two CPs was whether the primary hydroxyl group at C6 had been fully propionylated or not. Dimethyl phosphate, dimethyl thiophosphate, diethyl phosphate, or diethyl thiophosphate was introduced into the residual hydroxyl positions of the CPs. Chemical composition of the respective derivatives was characterized by elemental analysis and a combined use of saponification and HPLC quantification of the released propionic acid. Their thermal properties were investigated by DSC and TGA, and an intermediate residue of the pyrolysis was also examined by FT-IR spectroscopy. From the thermal degradation measurements using TGA, the C6-O phosphorylation was found to noticeably prevent the CP derivatives from weight loss in the pyrolysis process under dynamic air, i.e., providing them with a flame-resistance functionality, whereas the C2-O and C3-O phosphorylation did not give rise to such an appreciable resistance effect. A discussion was focused on the difference in pyrolysis mechanism between the phosphorylated CPs. However, most samples of the CP derivatives showed a clear T (g) considerably lower than the onset temperature of the thermal degradation. Thus we suggest that it is possible to design thermoplastic flame resistant/retardant materials based on cellulose, by controlling the substitution distribution of the phosphoryl and propionyl groups introduced.