1. Properties of microcrystalline cellulose.

　　Microcrystalline celluloseIt is a powdery or short-strip polymer compound with a particle size range of 20-200 meters. It is tasteless, highly fluid, but lacks fiber. Its molecules are mainly composed of crystalline regions, so its crystallinity is high, generally 55%-80%, and the crystal type is generally type I cellulose [5]. Studies have shown that the higher the crystallinity of cellulose, the better its toughness and tensile properties. Therefore, microcrystalline cellulose has good mechanical properties and is usually the preferred material for preparing composite materials [6-7]. Microcrystalline cellulose also has good biodegradability. Its degradation types are mainly divided into six types: acidic degradation, alkaline degradation, thermal degradation, microbial degradation, and mechanical degradation. Among them, acidic degradation is a common degradation method and can be used to prepare glucose and polymer compounds after acetylated nanocellulose. In terms of solubility, microcrystalline cellulose and molecules contain more molecules and hydrogen salts in the molecules, which makes it difficult to dissolve in other diluents, alkaline solvents, and some copper acid solvents.

　　2. Preparation of microcrystalline cellulose.

　　Microcrystalline celluloseIt mainly uses natural plants as raw materials, and its preparation process is mainly divided into three processes: ① pretreatment of raw materials and preparation of high-purity cellulose; ② degradation of cellulose to prepare microcrystalline cellulose; ③ drying of microcrystalline cellulose. The raw materials, preparation methods, and drying methods in the microcrystalline cellulose preparation process are mainly introduced as follows.

　　2.1 Raw materials.

　　Microcrystalline cellulose has abundant raw materials. Cotton wood pulp has a high cellulose content and has become the main raw material for the industrial production of microcrystalline cellulose [5]. However, with the increasing demand for wood and cotton in the furniture industry, pulp manufacturing industry, textile industry, and other industries, some other types of plant fiber raw materials have also been used to prepare microcrystalline cellulose, such as grass, bamboo, hemp [11], and some agricultural and forestry wastes [12-13], etc. Preparation of bambooMicrocrystalline cellulose[10] At the time, its yield, moisture content, and crystallinity were 83.3% ± 1.48%, 4.5% ± 0.5%, and 78%, respectively. The performance indicators are close to commercial-grade microcrystalline cellulose. The shape of microcrystalline cellulose prepared from banyan tree bark is rod-shaped, with a length of 90 ± 25 m and an average diameter of 17.1.3 m, and a crystallinity of 7 ± 5.8% [12]. Its initial heat cooling can be used as a low-quality wood material. Compared with wood materials, these materials have the advantages of a short growth cycle and low-quality raw materials.

　　2.2 Preparation methods.

　　Microcrystalline cellulose is a polysaccharide compound with small particle size and high crystallinity, composed of glucose units. The molecules are mainly composed of crystalline regions. In general, most amorphous cellulose raw materials are prepared by removing them through biological or chemical methods. Currently, acid hydrolysis and bioenzymes are common methods for preparing microcrystalline cellulose.

　　2.2.1 Acid hydrolysis method.

　　Acid hydrolysis of cellulose is a common method for preparing microcrystalline cellulose. It has the advantages of low cost, short time, high success rate, mature preparation method, and industrialized production. Its principle is that the β-1 and 4 glycosidic bonds break in the cellulose structure under acidic conditions, and the amorphous region is gradually removed. Cellulose molecules are degraded to the limiting degree of polymerization (15 to 375) to form microcrystalline cellulose. However, due to the special properties of acidity itself, the preparation of microcrystalline cellulose by acid hydrolysis also has disadvantages such as equipment corrosion, difficulty in wastewater treatment, and large water consumption during the preparation process.

　　The main factors affecting the acid hydrolysis method for preparing microcrystalline cellulose are: the type of acid, the acidolysis time, the solid-liquid ratio of cellulose to acid, the concentration of acid, and the auxiliary means used in the preparation process. These factors will affect the yield, particle size, thermal stability, degree of polymerization, purity, and crystallinity of microcrystalline cellulose.

　　The acid used in the preparation of the acid hydrolysis method can be liquid acids such as hydrochloric acid, sulfuric acid, and phosphoric acid, or solid acid phosphotungstic acid (HPW), among which hydrochloric acid and sulfuric acid are commonly used acids in the acid hydrolysis method. Compared with sulfuric acid, the polymerization and thermal stability of mixed wood microcrystalline cellulose prepared by hydrochloric acid.

　　The sex is higher. Using sulfuric acid, hydrochloric acid, nitric acid, and sulfuric acid/hydrochloric acid mixed acid to prepare reed MCC, respectively. The results show that the type of acid does not affect the form of MCC. The prepared MCC is short rod-shaped, but compared with hydrochloric acid (or nitric acid), the sulfuric acid preparationMicrocrystalline celluloseThe crystallinity is lower [16]. The results of preparing microcrystalline cellulose by dissolving cotton cloth with sulfuric acid of different concentrations show that the microcrystalline cellulose particles prepared with high-concentration sulfuric acid (64%) are finer and have lower modulus [17]. Compared with low-concentration sulfuric acid (35%), when using hydrochloric acid solution to prepare microcrystalline cellulose, the order of the influence degree of various factors affecting the microcrystalline cellulose acquisition rate from large to small is: acidolysis temperature > hydrochloric acid concentration > acidolysis time. Under better conditions,Microcrystalline celluloseThe yield can reach 54.34% [18].