The research on chitosan-based flame retardants mainly focuses on their flame retardant mechanisms, modification methods, embedding of flame retardant elements, and performance characteristics of composite materials, etc. The following is a specific analysis:
Flame retardant mechanism: Chitosan (CS) is a natural amino polysaccharide with a positive charge and can be directly used as a carbonizing agent for flame-retardant PLA composites. At high temperatures, CS undergoes ring-opening reactions and self-condenses in the matrix to form an aromatic ring cross-linked structure, that is, a carbon layer is generated in the condensed phase, which is conducive to inhibiting heat exchange in the matrix. Meanwhile, the amino groups in CS are released into the gas phase in the form of NH₃ during the thermal decomposition process. On the one hand, this can dilute the concentration of flammable gas; on the other hand, it promotes the formation of an expanded carbon layer. The expanded carbon layer has a better protective effect on the substrate than the ordinary carbon layer.
Modification methods: The functional groups of the CS structure mainly include C3-OH, C6-OH, C2-NH₂ and β(1-4) glycosidic bonds. Among them, -NH₂ and -OH have high activity and are prone to reactions. This enables CS to undergo chemical modification to form multifunctional CS derivatives, including chemical reactions such as acylation, esterification, etherification, Schiff alkalization, alkylation, quaternization and graft copolymerization. On the basis of maintaining the unique properties of CS itself, CS derivatives can be used to improve the solubility, static charge, porosity, mechanical strength, etc. of CS, thereby enhancing the performance of the material.
Embedding of flame-retardant elements: In addition to halogen-based flame retardants, compounds containing elements such as P, N, and Si can also be used in the development of flame retardants. The flame-retardant mechanism varies depending on the elements used. For instance, halogens capture free radicals, phosphorus induces dehydration and carbonization, and nitrogen promotes cross-linking. Meanwhile, different flame-retardant elements have different influences on the flame-retardant performance of materials. For example, bromides are suitable for plastics and rubber, and metal hydroxides are suitable for glass fibers and textiles. Therefore, the strategic selection of flame-retardant elements can significantly improve the flame-retardant performance of chitosan composite flame-retardant materials (FRCC).
The performance characteristics of composite materials: The basic properties ensure the feasibility of FRCC in practical applications, enabling it to maintain flame retardancy and inherent characteristics under external influences. Additional performance involves functional design to meet specific application requirements and expand its potential application scope. For instance, the addition of CS endows FRCC with excellent antibacterial properties, which helps to reduce bacterial infections and their spread, thereby promoting the hygiene and safety of the environment and products. In addition, in practical applications, FRCC may also be endowed with functions such as self-cleaning, UV resistance, insulation and conductivity, adsorption and self-repair to protect it from various external factors and expand its potential application range.
