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Views: 0 Author: Site Editor Publish Time: 2025-02-17 Origin: Site
The preparation of cellulose nanocrystals ( CNCs ) by ultrasonic means is a process of decomposing cellulose samples into nano-scale cellulose crystals by utilizing the cavitation effect of ultrasonic waves. The high-frequency vibration of ultrasonic waves causes extremely high temperature and pressure fluctuations in the liquid, thereby forming cavitation bubbles in the liquid. The shear force and high temperature effects generated by the cracking of the bubbles can effectively break the amorphous area of cellulose. Crystalline regions are exposed to form cellulose nanocrystals.
The core of ultrasonic method is the cavitation effect. The cavitation effect refers to the fluctuation that changes alternately between high and low pressures when ultrasonic waves propagate in liquids. During the low pressure cycle, gas solubles and tiny bubbles in the liquid will expand and eventually burst, resulting in local extremely high temperatures and pressures. This instantaneous high temperature and high pressure environment will cause chemical bonds between cellulose molecules to break, resulting in high crystallinity. Cellulose nanocrystals.
Frequency range : Usually between 20 kHz - 100 kHz .
Low frequency ( 20-40 kHz ) produces a strong cavitation effect, which is suitable for treating larger particles or more difficult-to-treat cellulose samples.
The cavitation effect generated by high frequency ( 40-100 kHz ) is relatively mild and is suitable for experiments with finer particle size or higher accuracy requirements.
Power density : Usually at 1 W/cm² - 10 W/cm² , a higher power density can enhance the cavitation effect and thus improve the decomposition efficiency, but may also lead to excessive degradation or the solution warms up too quickly.
The preparation of cellulose nanocrystals usually select plant-derived cellulose, such as wood, cotton, corn stalks, etc. In order to improve preparation efficiency and ensure high nanocrystal yields, raw materials need to be pretreated:
Removal of impurities : Chemical or physical methods are used to remove impurities such as lignin and hemicellulose, and chemical agents such as NaOH or H₂O₂ are usually used to remove them.
Mechanical crushing : Mechanically crush the raw materials to make their particles thinner and their surface area larger, thus helping the effect of ultrasonic waves.
Cellulose is usually dispersed in water or an aqueous solution containing a small amount of organic solvent, and the solution concentration is usually between 0.5% - 5% ( w/v ). The common solvent is water, but ethanol or acidic solution can also be added as needed to improve dispersion.
Ultrasonic devices are usually composed of ultrasonic generators, transducers and probes to control the degree of cellulose decomposition by adjusting the power, frequency and processing time of the ultrasonic waves. Commonly used experimental parameters include:
Ultrasonic frequency : Select a frequency of 40 kHz or 60 kHz for processing. Lower frequencies help to produce a stronger cavitation effect, while higher frequencies create a more subtle cavitation bubble.
Ultrasonic power : The power range is usually 300 W - 500 W. The higher the power, the stronger the cavitation effect is generated and the greater the energy consumption. In experiments, is often used 400 W of power for processing to obtain higher yields and crystallinity.
Processing time : usually between 30minutes - 60 minutes . Longer processing times help to obtain smaller particle size CNCs , but may also lead to particle degradation or increased solution temperature. The common experimental time is 30minutes , and the crystallinity of the cellulose sample can reach 70%.。
Ultrasonic solutions usually require post-treatment to remove unreacted larger particles and other impurities. Commonly used separation methods include:
Centrifugal separation : Centrifuge treatment at a speed of 10,000 rpm for 10-15 minutes , and large unreacted particles were separated to obtain a dispersion with higher purity of cellulose nanocrystals.
Filtration : Filter the solution using a 0.2 μm filter membrane to remove larger particles to obtain fine cellulose nanocrystals.
The obtained cellulose nanocrystals usually require drying, and common methods are freeze-drying or spray-drying . The resulting CNC powder is easy to store and transport by drying.
The following is a typical experimental parameter for the preparation of cellulose nanocrystals by ultrasonic method:
Cellulose concentration : 1% ( w/v )
Ultrasonic frequency : 40 kHz
Power : 400 W
Processing time : 30minutes
Temperature : 20-30°C
Particle size : After treatment with this condition, the particle size range of cellulose nanocrystals is usually 30-70 nm。
Crystallinity : The crystallinity of CNC after treatment can reach about 70%。
Efficient and environmentally friendly : Compared with traditional acid hydrolysis, ultrasonic method does not require the use of strong acids or toxic chemical reagents, so it is more environmentally friendly and has a short reaction time ( 30it can be prepared within minutes ).
可控性强：超声波的功率、频率和处理时间可调，能够精确控制纤维素纳米晶体的粒径和结晶度。 For example, by selecting parameters of 40 kHz and 500 W , CNC with particle size 40 nm can be obtained。
Wide scope of application : This method can not only be applied to wood cellulose, but also can treat cellulose from other plants such as cotton, straw, corn stalks, etc.
Energy Consumption : The energy consumption of ultrasonic methods is high, especially in large-scale production, which may affect cost-effectiveness. For example, it takes about 30-60 minutes to process 100 g of cellulose , while high-power devices consume more energy.
Temperature control : The heat generated during ultrasonic treatment may affect the stability of the reaction. Therefore, it is necessary to control the temperature to avoid overheating leading to cellulose degradation.
The effect of ultrasonic method in preparing CNC is affected by a variety of factors, and the following are the main influencing factors:
Power density : The power density ( 1 W/cm² - 10 W/cm² ) directly determines the intensity of the cavitation effect. Too high power can lead to excessive degradation, while too low may not be effective in producing nanoscale crystals.
Frequency and time : Different frequencies ( 40 kHz - 60 kHz ) and treatment time ( 30 - 60 minutes ) will affect the degree of cellulose decomposition and the particle size distribution of the crystal. Ultrasonic treatment at higher frequencies can obtain finer CNC , but the cavitation effect is weaker.
Solution concentration : The solution concentration is usually between 0.5% and 5% . Higher concentrations help improve reaction efficiency, but may also make it difficult for cellulose particles to disperse evenly.
Ultrasonic method is an efficient and green preparation method. By adjusting the frequency, power and time of ultrasonic waves, the particle size and crystallinity of cellulose nanocrystals can be accurately controlled. Despite the significant advantages of this method, there are still challenges such as energy consumption and temperature control that need to be optimized and improved in practical applications.
