Chitosan (CS) is prepared by deacetylation of chitin, which has the advantages of regeneration and good biocompatibility. CS is a natural amino polysaccharide with positive charge, which can be directly used as the charring agent of flame retardant PLA composites. Ring opening reaction will occur at high temperature and self condense in the matrix to form an aromatic ring cross-linking structure, that is, a carbon layer is formed in the condensed phase, which is conducive to inhibiting the heat exchange in the matrix. At the same time, the amino group in CS is released to the gas phase in the form of NH3 during thermal decomposition. On the one hand, it can dilute the concentration of combustible gas, on the other hand, it can promote the formation of expanded carbon layer. The expanded carbon layer has better matrix protection than ordinary carbon layer.
In general, CS and acid source (such as app) form an intumescent flame retardant system. The products generated by acid source during thermal decomposition can promote the dehydration and carbonization of chitosan. In addition, because there are many active groups in its structure, it can also be modified to optimize its flame retardant performance.
CS has excellent flame retardancy in PLA. Researchers try to add CS and halogen-free flame retardant into the matrix by blending to improve the flame retardancy of the material. App has a good flame retardant effect in PLA. Zhang et al. Prepared core-shell flame retardant by LBL technology with app as the core and cationic CS and anionic alginic acid (AA) as the shell APP@CS @AA ⁃ NBL (BL represents double layers, n represents the number of double layers, and 1BL consists of a pair CS@AA The flame retardant material was prepared by melt blending with PLA, which was compared with pure PLA, The rphrr of the composite decreased significantly. When the number of layers was only 3bl, the limiting oxygen index of the material increased to 30.6%, and the carbon residue rate increased from 1.5% (pure PLA) to 13.2%. This is because CS and AA can be used as carbon sources to form IFR system with app. Because CS and AA have polyhydroxy structure, the phosphoric acid and polyphosphate released by app during thermal decomposition promoted the dehydration and carbonization of CS and AA, The carbon layer becomes dense and continuous, which can effectively prevent the escape of combustible gas and inhibit the heat and oxygen exchange, so as to improve the flame retardant performance of the material.
In addition to the above-mentioned intumescent flame retardant system, CS modified system has higher thermal stability and better heat barrier effect than ordinary carbon layer, and inhibits the spread of heat and combustible gas. Current reports on CS flame retardant PLA Less, but because CS has good char-forming properties and environmental protection, the research of flame-retardant PLA system containing CS will have a good development prospect in the future
