Cellulose (the chemical structure is shown in Figure 1) is a cell wall component. In short, cellulose is composed of glucose molecules β⁃ 1,4 ⁃ linear macromolecule connected by glycosidic bond, with molecular formula of (C6H10O5) (n, where n is the degree of polymerization), which is widely distributed in nature and mainly exists in the cell wall of higher plants, bacteria, algae and fungi. The thermal degradation process of cellulose can be roughly divided into the following four stages. In the first stage, physical dehydration occurs at low temperature to remove the crystal water from the cellulose; In the second stage, chemical dehydration occurs at about 150 ℃ to produce water and dehydrated cellulose. The formation of water is conducive to accelerating the hydrolysis of glycosidic bonds and promoting the degradation of cellulose; With the increase of temperature, the thermal decomposition and carbonization reaction begin at 240 ℃ in the third stage to produce liquid product tar and carbonaceous intermediate products. At the same time, dehydrated cellulose further reacts to produce carbon monoxide, carbon dioxide and water vapor; In the fourth stage, carbon aromatization and cross-linking occur above 400 ℃ to form coke residue. It should be noted that under high temperature conditions, the reaction tends to produce tar and inhibit coke formation. However, rich modification technologies are conducive to improve the flame retardant properties of cellulose at high temperature.
Cellulose can be hydrolyzed by inorganic acid to form cellulose with a low degree of polymerization and a certain degree of crystallinity, called microcrystalline cellulose (MCC). In addition, cellulose can also be hydrolyzed to form nanocrystalline cellulose (NCC). The physical properties of MCC and NCC are significantly different, and NCC has the characteristics of high crystallinity. In recent years, MCC and NCC have also been used for flame-retardant PLA. Yin et al. newly designed a new type of green hybrid flame-retardant system, using CNF as a surface modifier, wound on APP through hydrogen bond interaction during ball milling, adding the prepared APP@CNF to PLA, The composite material was prepared by the melt blending method. The results showed that compared with pure PLA, the limiting oxygen index of PLA composite material with only 5.0% ammonium polyphosphate@CNF increased to 27.5%, and UL 94 reached V-0. Level, and the impact strength has increased by 54%. The increase in flame retardancy and mechanical properties is due to the synergy between APP and CNF and the improved dispersion of ammonium polyphosphate @CNF in the PLA matrix. Researchers have found that CNF can also improve flame retardancy through surface modification techniques such as silanization, amidation, esterification, etherification, and polymerization. Cellulose containing a large amount of natural hydroxyl structure has potentially good char-forming properties, but its thermal stability is not good and cannot meet the processing requirements of PLA. Therefore, the flame retardant system is still in the basic research stage and has not yet been industrialized.
