Catalyzing carbonization of poly(L-lactide) by nanosized carbon black combined with Ni2O3 for improving flame retardancy

Catalyzing the carbonization of a polymer itself to form a protective carbonaceous layer has attracted tremendous interest in the flame retardancy of polymers, but the efficiency of this strategy was largely limited to polymers containing heteroatoms within their backbone. In this study, a novel type of combined catalyst, consisting of nanosized carbon black (CB) and Ni2O3, was found to efficiently catalyze the carbonization of poly(L-lactide) (PLA) for the first time. The combined catalyst was more efficient than CB or Ni2O3 alone to enhance the char yield of PLA and improve its char layer structure, which greatly improved the flame retardancy of PLA. The catalytic pyrolysis for PLA composites and model carbonization experiments for their degradation products were carried out to investigate the carbonization process of PLA during combustion. The results indicated that the carbonization mechanism was attributed to the combined effect of the combined catalysts: CB catalyzed the degradation of PLA to selectively produce more aldehydes and ketones with lower carbon numbers, then these carbonyl compounds were dehydrogenated (and/or dehydrated) and transferred into carbon products catalyzed by the combination of CB and Ni catalysts.