Phosphorylated chitosan‐cobalt complex: A novel green flame retardant for polylactic acid

The exploration of green flame retardants is of great significance for fire safety, ecological environment protection, and resource saving. Herein, a novel phosphorus containing chitosan‐cobalt complex (CS‐P‐Co) was successfully synthesized for the first time and then introduced into polylactic acid matrix. Owning to the formation of cobalt salts, the CS‐P‐Co exhibits good thermal stability and catalytic char effect. The as‐fabricated biodegradable polylactic acid/CS‐P‐Co composites with small amounts of CS‐P‐Co (≤ 4.0 wt%) show remarkably improved flame retardancy, such as decreased peak heat release rate and total heat release by 23.0% and 20.0%, respectively. Scanning electron microscopy, Thermogravimetric analysis, and Raman results demonstrate that compact graphitized char layers are formed on the composite surface during combustion, attributed to the catalytic effect of CS‐P‐Co. The char inhibits diffusion of heat, mass, and oxygen, which plays a key role in the flame retardancy enhancement.