Preparation process of nanocellulose

Nanocellulose (NC) is a nano-scale fiber made from physical, chemical or biological treatment of natural plant cellulose. It has a unique nanostructure (fiber size is usually between 1-100 nanometers), a high specific surface area (about 300-1000 m²/g), excellent mechanical properties (tensile strength can reach 200-300 MPa), and has a high specific surface area (about 300-1000 m²/g), excellent mechanical properties (tensile strength can reach 200-300 MPa), and has Good biodegradability and environmental friendliness. Nanocellulose is widely used in composite materials, coatings, electronic equipment, medicine, food packaging and other fields, and is one of the hot topics in the field of materials science.

The main types of nanocellulose:

Nanocellulose whiskers (CNC) : highly crystalline nanocellulose, with crystalline rod-like or fibrous structures.

Nanocellulose fiber (CNF) : Long-chain, non-crystalline fiber structure, with good water solubility and flexibility.

The following are several common nanocellulose preparation processes. The selection of each process directly affects the performance, cost and application fields of the final product.

1. Acid hydrolysis method (acid solution method)

Acid hydrolysis method uses concentrated sulfuric acid or other strong acids to hydrolyze the amorphous part of the cellulose, retaining the nanocellulose whiskers with high crystallinity. This method can efficiently prepare nanocellulose whiskers with high crystallinity and strong rigidity, and is widely used in composite materials and reinforced coatings.

Process flow:

step	Specific operations
Preprocessing	Lignocin and hemicellulose were removed using sodium hydroxide solution to obtain pure cellulose.
Acid hydrolysis	Cellulose and concentrated sulfuric acid (60-70%) react at a temperature of 50-70°C for several hours.
Neutralization and cleaning	Neutralize the acid residue with sodium hydroxide solution, wash with deionized water to remove residual acid.
Dry and disperse	The final nanocellulose is obtained by spray drying or freeze drying after ultrasonic dispersion.

advantage:

High crystallinity : Acid hydrolysis can produce nanocellulose whiskers with high crystallinity and strong rigidity, with excellent performance.

Uniform structure : The reaction conditions can be controlled during the preparation process to obtain highly uniform nanofibers.

shortcoming:

Acid wastewater treatment : The waste acid generated during acid hydrolysis needs to be treated to increase environmental pressure.

High energy consumption : High temperature conditions are required during acid hydrolysis and consumes more energy.

Performance metrics:

Fiber Size : 2-5 nm (diameter), hundreds of nm (length).

Crystallinity : 70-90%.

Tensile strength : 200-300 MPa.

2. Mechanical method (high pressure homogeneity method)

The mechanical method is to dissociate the cellulose slurry into nanoscale fibers through high-pressure homogenization equipment or ultra-high shearing equipment. This method does not rely on chemical reagents, meets the requirements of green chemistry, and is suitable for large-scale production.

Process flow:

step	Specific operations
Preprocessing	Perform alkali treatment or enzymatic dissolution of cellulose to remove lignin and hemicellulose and improve looseness.
High pressure homogeneity	The cellulose slurry is dissociated into nanofibers under a pressure of 200-300 MPa through a high-pressure homogenizer.
Dispersion and drying	Use an ultrasonic dispersion device to prevent fiber agglomeration and then spray-dry or freeze-dry.

advantage:

Environmentally friendly and green : No chemical reagents, easy to operate, suitable for large-scale industrial production.

High production efficiency : suitable for continuous production lines, high production efficiency.

shortcoming:

High energy consumption : High voltage homogenization process requires higher energy input.

Poor dispersion : Nanofibers may agglomerate and require additional treatment.

Performance metrics:

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

Tensile strength : 100-200 MPa.

Specific surface area : 300-800 m²/g.

3. TEMPO oxidation method

TEMPO oxidation method oxidizes part of the hydroxyl group of cellulose into carboxyl groups through TEMPO catalyst and sodium hypochlorite to produce nanocellulose with good dispersion and water-soluble, which is particularly suitable for the preparation of highly functional nanocellulose.

Process flow:

step	Specific operations
Oxidation reaction	The TEMPO catalyst works together with sodium hypochlorite to oxidize the hydroxyl group on the surface of cellulose under pH 10-11.
Neutralization and cleaning	Neutralize the oxidation by-products, remove the unreacted TEMPO catalyst, and wash it multiple times with deionized water.
Dispersion and drying	Dispersion treatment is performed using an ultrasonic dispersion device, followed by spray drying or freeze drying.

advantage:

Surface functionalization : Carboxylic groups introduced during oxidation enhance the water solubility and compatibility with other materials.

Good dispersion : The oxidized nanocellulose has good dispersion and is suitable for aqueous coatings and drug delivery systems.

shortcoming:

Higher cost : TEMPO catalysts and oxidants have higher costs, increasing production costs.

Reaction control requirements are high : reaction conditions need to be strictly controlled to ensure efficient oxidation.

Performance metrics:

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

Surface carboxyl content : 2-3 mmol/g.

Water solubility : > 99%.

4. Bacterial fermentation method

Bacterial fermentation method synthesizes bacterial cellulose (BC) through bacteria (such as Komagataeibacter xylinus ), which has high purity and excellent biocompatibility. This method is suitable for the preparation of high-quality nanocellulose, especially suitable for the pharmaceutical and food fields.

Process flow:

step	Specific operations
Culture medium preparation	Prepare culture medium containing glucose, nitrogen sources, mineral salts, etc. to support bacterial growth.
Fermentation process	Bacteria are fermented under suitable temperature (30-32°C) and pH (4-5) conditions to synthesize bacterial cellulose.
Extraction and purification	Remove bacteria and impurities from the fermentation product to obtain pure bacterial cellulose.
Dry and preserve	Bacterial cellulose is obtained by freeze-drying or spray-drying, which is convenient for long-term storage.

advantage:

High purity and biocompatibility : The cellulose obtained by bacterial fermentation has high purity and good biodegradability, and is suitable for the pharmaceutical and food fields.

Complex chemical reagents are required : the reaction process is non-toxic by-products and is environmentally friendly.

shortcoming:

Long production cycle : The fermentation process takes several days or weeks, and the production cycle is relatively long.

High production cost : The bacterial culture and purification process are more complicated, resulting in higher costs.

Performance metrics:

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

Tensile strength : 100-200 MPa.

Specific surface area : 200-500 m²/g.

Summarize

As the application field of nanocellulose continues to expand, the choice of different preparation processes has an important impact on the performance and cost of the final product. Acid hydrolysis method is suitable for nanocellulose with high crystallinity and strong rigidity.