Preparation method of sisal nanocellulose

Nanocellulose can be classified according to its preparation method. Different preparation methods are named differently. At the same time, its morphology and performance vary with the preparation method used. The preparation methods of nanofibers include chemical methods (acid hydrolysis, enzyme hydrolysis), physical methods (high pressure homogenization, ball milling, high speed stirring, etc.), chemical+Physical method (TIME-Oxidation methodEach preparation method has its advantages and disadvantages. Below, we will analyze their strengths and weaknesses, along with solutions for commercial applications. We will focus on the morphology, surface charge, energy consumption, yield, and other aspects of the prepared nanocellulose that are relevant to its application.

1Sulfuric acid hydrolysis method

The prepared nanocellulose is called cellulose nanowhiskers.CNW、CNCorNCCIts diameter varies depending on its source.，For example, the average diameter of sisal cellulose nanowhiskers is9 nm, the average length is170 nm, while the diameter of the one made with bacterial cellulose is about tens of nanometers.It is much larger than cellulose nanowhiskers from other sources. Its preparation process is based on the selective hydrolysis of the amorphous region of cellulose by acid, leaving the crystalline region of cellulose to obtain whisker-like nanomaterials. Among them, the acid can selectively hydrolyze the amorphous region of cellulose because the amorphous region has a loose structure and the acid solution can easily enter this region; while the crystalline region of cellulose has a dense structure, and the cellulose molecular chains form a regular arrangement and hydrogen bond interaction, which makes it impossible for acid solutions of normal concentration to enter its interior and cut off its crystalline region. Therefore, the cellulose prepared by acid hydrolysis isCNWorCNCThe structure has a very high degree of crystallinity, and the morphology retains the structure of the crystalline region of the raw material cellulose. At the same time, due to the high degree of crystallinity of the crystalline region, the cellulose chains are highly oriented and regular, and a large number of hydrogen bonds are formed between the molecular chains, soCNWorCNCIt has very excellent mechanical properties and is an ideal reinforcing material.

However, it also has the following disadvantages: first, the yield of its preparation is low; second, it is difficult to disperse after drying; and third, its dispersibility in hydrophobic polymers is very poor. These problems restrict the development of cellulose nanowhiskers. Regarding the yield of cellulose nanowhiskers, in the existing literature reports, the yield of cellulose nanowhiskers prepared by acid hydrolysis is no more than40%.For example,Brito^[73]Wait for it to pass65 wt%The highest yield of cellulose nanowhiskers prepared by hydrolysis with sulfuric acid is30%Regarding the problem of redispersion after drying, there are multiple hydroxyl groups on the cellulose molecular chain, and it is very easy for hydrogen bonds to form between the molecules.CNWWhen in water, they form liquid crystal arrangements, and after drying, they are arranged more closely together, intensifying the interaction between them.Therefore, the current nanocellulose is dispersed in aqueous solution and can only be purchased7-10 wt%Left and rightDispersion form. The dispersion of nanopowders is crucial in reinforced polymers. Managing the interfacial interaction with the matrix material significantly impacts the performance of the modified and reinforced polymer. To address the dispersion issue, surface modification is the preferred method.Dufresne^[56]et al. studied the use of modified ramie cellulose nanowhiskers to reinforce polyethylene. They first reacted acyl chlorides of varying fatty chain lengths with the cellulose nanowhiskers and then extruded and blended them with polyethylene. Their results showed that the modified cellulose nanowhiskers improved their dispersibility in polyethylene. Furthermore, the modified composites exhibited significantly higher elongation at break than unmodified cellulose nanowhiskers-reinforced composites.

2Mechanical Method

The microfibrillated cellulose with a diameter of tens to hundreds of nanometers and a length of several micrometers is obtained by mechanical treatment of plant fibers such as disc grinding and high-pressure averaging.MFCorNFCThe principle of mechanical preparation of nanocellulose is to break down natural plant fibers into nanofibers by mechanical shearing. The advantages of mechanical preparation are simple process and no organic pollutants.(Such as acid, alkali, organic solvent, etc.)However, its disadvantages are also obvious. The preparation process is energy-intensive, the nanofibers produced are large in diameter, and the surface lacks strong hydrophilic groups. Nanocellulose produced by mechanical methods is expected to be used in materials such as papermaking reinforcements and lithium battery separators.

3Chemical+Mechanical Method