Preparation Processes of Nanocellulose

Nanocellulose (NC) is a nanometric fiber derived from natural plant cellulose through physical, chemical, or biological treatments. It has a unique nanostructure (with fiber dimensions typically ranging from 1-100 nanometers), a high specific surface area (around 300-1000 m²/g), excellent mechanical properties (with tensile strength reaching 200-300 MPa), and good biodegradability and environmental friendliness. Nanocellulose is widely used in composite materials, coatings, electronics, pharmaceuticals, food packaging, and other fields, and it is currently a research hotspot in material science.

Main Types of Nanocellulose:

Cellulose Nanocrystals (CNC): Highly crystalline nanocellulose, presenting a crystalline rod-like or fibrous structure.

Cellulose Nanofibrils (CNF): Long-chain, non-crystalline fiber structure, exhibiting better water solubility and flexibility.

Below are several common preparation processes for nanocellulose, each of which directly impacts the final product’s performance, cost, and application areas.

1. Acid Hydrolysis Method (Acid Hydrolysis)

The acid hydrolysis method uses concentrated sulfuric acid or other strong acids to hydrolyze the non-crystalline parts of cellulose, preserving the high crystallinity of cellulose nanocrystals. This method is highly efficient in preparing high-crystallinity, rigid cellulose nanocrystals, which are widely used in composite materials and enhanced coatings.

Process Flow:

Step	Specific Operation
Pretreatment	Use sodium hydroxide solution to remove lignin and hemicellulose, obtaining pure cellulose.
Acid Hydrolysis	Cellulose is reacted with concentrated sulfuric acid (60-70%) at 50-70°C for several hours.
Neutralization & Washing	Neutralize the acidic residue with sodium hydroxide solution, wash with deionized water to remove residual acid.
Drying & Dispersion	After ultrasonic dispersion, the final nanocellulose is obtained through spray drying or freeze-drying.

Advantages:

High Crystallinity: Acid hydrolysis produces cellulose nanocrystals with high crystallinity and excellent performance.

Uniform Structure: Reaction conditions can be controlled during the preparation process to achieve highly uniform nanofibers.

Disadvantages:

Acid Waste Treatment: Acidic waste generated during the hydrolysis process requires treatment, increasing environmental pressure.

High Energy Consumption: High temperature conditions are required during acid hydrolysis, consuming more energy.

Performance Indicators:

Fiber Size: 2-5 nm (diameter), several hundred nm (length).

Crystallinity: 70-90%.

Tensile Strength: 200-300 MPa.

2. Mechanical Method (High-Pressure Homogenization)

The mechanical method uses high-pressure homogenization equipment or ultra-high shear devices to break down cellulose slurry into nanoscale fibers. This method does not rely on chemical reagents and is in line with green chemistry requirements, making it suitable for large-scale production.

Process Flow:

Step	Specific Operation
Pretreatment	Alkali treatment or enzymatic hydrolysis of cellulose to remove lignin and hemicellulose, improving looseness.
High-Pressure Homogenization	The cellulose slurry is subjected to pressure of 200-300 MPa using a high-pressure homogenizer, breaking down into nanofibers.
Dispersion & Drying	Ultrasonic dispersion is used to prevent fiber agglomeration, followed by spray drying or freeze-drying.

Advantages:

Environmentally Friendly: No chemical reagents, simple operation, suitable for large-scale industrial production.

High Production Efficiency: Suitable for continuous production lines, offering high production efficiency.

Disadvantages:

High Energy Consumption: The high-pressure homogenization process requires significant energy input.

Poor Dispersion: Nanofibers may agglomerate, requiring additional treatment.

Performance Indicators:

Fiber Size: 20-50 nm (diameter), several micrometers (length).

Tensile Strength: 100-200 MPa.

Specific Surface Area: 300-800 m²/g.

3. TEMPO Oxidation Method

The TEMPO oxidation method uses TEMPO catalysts and sodium hypochlorite to oxidize some hydroxyl groups on the cellulose surface into carboxyl groups, producing nanocellulose with good dispersion and water solubility. This method is particularly suitable for the preparation of highly functionalized nanocellulose.

Process Flow:

Step	Specific Operation
Oxidation Reaction	TEMPO catalyst and sodium hypochlorite work together to oxidize hydroxyl groups on cellulose under pH 10-11 conditions.
Neutralization & Washing	Neutralize oxidation by-products, remove unreacted TEMPO catalyst, and wash thoroughly with deionized water.
Dispersion & Drying	Use ultrasonic dispersion for treatment, followed by spray drying or freeze-drying.

Advantages:

Surface Functionalization: The carboxyl groups introduced during oxidation enhance the water solubility and compatibility of nanocellulose with other materials.

Good Dispersion: Oxidized nanocellulose exhibits good dispersion, making it suitable for water-based coatings and drug delivery systems.

Disadvantages:

Higher Cost: The cost of TEMPO catalysts and oxidizing agents is higher, increasing production costs.

Strict Reaction Control: Reaction conditions need to be carefully controlled to ensure efficient oxidation.

Performance Indicators:

Fiber Size: 5-10 nm (diameter), 200-500 nm (length).

Surface Carboxyl Content: 2-3 mmol/g.

Water Solubility: > 99%.

4. Bacterial Fermentation Method

The bacterial fermentation method uses bacteria (such as Komagataeibacter xylinus) to synthesize bacterial cellulose (BC), which has high purity and excellent biocompatibility. This method is suitable for preparing high-quality nanocellulose, especially for pharmaceutical and food applications.

Process Flow:

Step	Specific Operation
Culture Medium Preparation	Prepare a culture medium containing glucose, nitrogen sources, and mineral salts to support bacterial growth.
Fermentation Process	Bacteria ferment at suitable temperatures (30-32°C) and pH (4-5) to synthesize bacterial cellulose.
Extraction & Purification	Remove bacteria and impurities from the fermentation product to obtain pure bacterial cellulose.
Drying & Preservation	Use freeze-drying or spray-drying to preserve bacterial cellulose for long-term storage.

Advantages:

High Purity and Biocompatibility: Bacterial cellulose produced through fermentation is highly pure and biodegradable, making it suitable for pharmaceuticals and food applications.

No Toxic By-products: The process does not produce toxic by-products and is environmentally friendly.

Disadvantages:

Long Production Cycle: The fermentation process takes several days or weeks, resulting in a longer production cycle.

High Production Cost: Bacterial culture and purification processes are complex, leading to higher production costs.

Performance Indicators:

Fiber Size: 30-80 nm (diameter), several micrometers (length).

Tensile Strength: 100-200 MPa.

Specific Surface Area: 200-500 m²/g.

Summary

As the application areas of nanocellulose continue to expand, the choice of preparation process significantly influences the final product’s performance and cost. The acid hydrolysis method is suitable for producing high-crystallinity, rigid nanocellulose.