Lignin exists in plant cell wall and forms the basic skeleton of plants together with cellulose and hemicellulose. It is a polyhydroxy aromatic compound, which meets the requirements of carbon source as intumescent flame retardant. The old bond fracture and new bond formation will occur in the combustion process of lignin. The pyrolysis process can be roughly divided into the following three stages. The first stage is free water volatilization; In the second stage, from about 120 ℃, the weak valence bonds around the benzene ring break and the volatile components recombine; In the third stage, when the temperature reaches 800 ℃, the benzene ring is cracked, volatilized and polymerized into polynuclear aromatic compounds. With the further increase of temperature, the new aromatic compounds undergo polycondensation and carbonization. The results show that lignin has high carbonization ability at high temperature and almost no carbonization in ordinary combustion process. However, its structure contains a variety of functional groups, such as methoxy, alcohol hydroxyl, phenol hydroxyl, benzene, aldehyde and carbonyl, which provides rich active sites for further chemical modification and is conducive to improving its flame retardant properties. Costes et al. Studied the flame retardant effect of kraft paper lignin and organic solvent lignin in PLA. Compared with traditional lignin, they have the advantages of moderate relative molecular weight and rich reactive groups. Gordon bil et al acetylated kraft lignin (KL), prepared modified lignin acetylated kraft lignin (AKL), and melt blended it with PLA to prepare composites and explore their thermal properties. A large number of experiments show that modified lignin can significantly improve the flame retardant properties of PLA. Because lignin has unique aromatic structure and high carbonization ability, lignin and its derivatives have great flame retardant potential in PLA. At present, most of the reports on lignin in PLA are to explore its impact on the flame retardant and thermal properties of PLA. However, some studies have found that lignin based flame retardants can improve the flame retardant, mechanical properties and UV resistance of PLA at the same time, which provides a new idea for the development of functional flame retardant PLA composites in the future.
