Accelerated Dewatering of Cellulose Nanofibers for Paper Production

Sonication in combination with lactic acid (LA) modification is an eco-friendly approach to accelerate the dewatering process of cellulose nanofibers (CNFs) for paper production. Cellulose nanofibers are known for their high water retention, causing significant delays in the drainage process, a critical step in nanopaper preparation and other nanocellulose applications. The method reduces the draining time by 75%, from 45 minutes to 10 minutes, through the esterification of cellulose hydroxyl groups by lactic acid under sonication. This modification also lowers suspension viscosity and proves to be industrially scalable, offering a practical solution to enhance the efficiency and production rate of cellulose nanofiber-based products.

Rapid dewatering of lactic acid modified cellulose nanofibers under sonication
(Study and image: ©Sethi et al., 2018)

Enhanced Dewatering Efficiency of Cellulose Nanofibers via Ultrasonic Lactic Acid Modification

Cellulose nanofibers (CNFs) are celebrated for their numerous benefits in paper production, such as enhancing strength and modifying rheology. However, one significant drawback is the inordinate amount of time required to drain water from cellulose nanofiber suspensions, a critical step in preparing nanopapers. This challenge not only impacts nanopaper production but also hinders the industrial processing of nanocellulose into other finished products. The prolonged drainage time is primarily due to the high water retention by cellulose nanofibers, which is a bottleneck in the commercialization of cellulose nanofiber-derived products.
Below, we present you the results of a scientific study by Sethi and colleagues (2018), who developed an ultrasonically-assisted lactic acid modification of cellulose nanofibers, which leads to a 75% faster dewatering of CNFs and improved material functionalities.

Ultrasonic Modification of Cellulose Nanofibers with Lactic Acid

To address this issue, an eco-friendly, water-based approach involving sonication energy and lactic acid (LA) has been developed to modify the surface of cellulose nanofibers. This method significantly accelerates the dewatering process, reducing the drainage time by up to 75%.
The modification process involves diluting cellulose nanofiber suspensions to a concentration of 0.35 wt.% and adding lactic acid in various amounts relative to the cellulose nanofiber dry content (0.5 times, 1 time, 5 times, and 10 times of the cellulose nanofiber dry content in suspension). The nanofibers, water, and lactic acid are mixed using a high-speed stirrer at 1500 rpm for 5 minutes and then sonicated with a Hielscher UP400S sonicator equipped with a titanium tip (22 mm in diameter). The sonication is conducted at various energy levels, with a maximum imparted energy of 600 J/ml, corresponding to 10 minutes of sonication.

Impact on Draining Time

The ultrasonic-lactic acid modification has a profound impact on the draining time of cellulose nanofiber suspensions. The reference suspension takes approximately 45 minutes for dewatering. After sonication-assisted lactic acid modification, this time is reduced to 10 minutes, representing a 75% improvement. Even with mild sonication (5 J/ml), the draining time is halved to 23 minutes. The draining time decreases with increasing sonication energy, eventually reaching a plateau at around 10 minutes.
This improvement is attributed to the replacement of hydrophilic hydroxyl groups of cellulose with hydrophobic moieties of lactic acid. The hydroxyl groups are primarily responsible for high water retention. Lactic acid, with its carboxylic group, can participate in an esterification reaction with the hydroxyl groups of cellulose under sonication. Sonication creates minuscule vacuum cavities in the liquid medium, which collapse to produce extreme conditions (5000 K temperature and 1000 atm pressure), sufficient to induce chemical reactions, including esterification.

Sonication energy vs draining time (in minutes) for nanopaper preparation (corresponding to CNF(1)LA sample).
100 J/ml of sonication energy corresponds to 100 seconds of sonication, etc.
(Study and image: ©Sethi et al., 2018)

Viscosity Reduction of Cellulose Nanofiber Suspensions

Sonication energy also contributes to a decrease in the viscosity of the cellulose nanofiber suspensions. The viscosity is inversely proportional to the sonication energy, with the lowest viscosity observed at 600 J/ml, followed by 300 J/ml and 60 J/ml. This reduction in viscosity further aids in improving the dewatering efficiency.

Industrial Relevance of Ultrasonic Cellulose Nanofiber Modification

The ultrasonic-lactic acid modification method is not only effective but also compatible with industrial applications. Hielscher state-of-the-art sonicators capable of handling large-volume processing can efficiently complete the task, making this method suitable for large-scale operations.
Additionally, this approach allows for the incorporation of up to 10 wt.% cellulose nanofiber in paper sheets, with dewatering completed in just 2 minutes, compared to 23 minutes for unmodified cellulose nanofibers. This significant improvement can revolutionize the use of cellulose nanofibers in papermaking, overcoming one of the major hurdles in their commercialization.

Sonicators for Cellulose Nanofibers and Paper Production

The table below gives you an indication of the approximate processing capacity of our probe-type sonicators:

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