(652b) Assessing the Dispersion Influence of Cellulose Nanofibers on Papermaking Applications

Nanocellulose, a unique and promising natural material extracted from native cellulose, has gained much attention for its use as reinforcing agent in several matrix, because of its remarkable mechanical and physical properties, special surface chemistry and excellent environmental properties (biocompatibility, biodegradability and low toxicity). Generally, the family of nanocellulose can be divided in three types, (1) cellulose nanocrystals (CNC), with the synonyms of nanocrystalline cellulose, cellulose whiskers, rod-like cellulose microcrystals; (2) cellulose nanofibers (CNF), with other designations such as nanofibrillated cellulose or cellulose nanofibrils; and (3) bacterial cellulose (BC), which refers to nanocellulose produced by bacteria ¹.

The sources for CNC and CNF extraction are wood, cotton, hemp, flax, wheat straw, sugar beet, potato tuber, mulberry bark, ramie, algae, and tunicin ^2-4. Acid hydrolysis is commonly performed for the extraction of CNC from native cellulose, through the removal of amorphous regions and preservation of highly-crystalline structure ⁵. Some of the advantages that CNC suspensions are the self-assembly, very high strength due to the high crystallinity, high functionalization for its inherently high surface area, unique optical properties and lightweight ⁶. Recently, CNC have become an extremely important class of nanomaterials to reinforce pulp suspensions ^7-9. Sulfuric acid hydrolysis produced stable CNC suspensions due to sulfate groups formed at the nanocrystal surface ¹⁰. The presence of these negatively charged groups induces the formation of a negative electrostatic layer covering the nanocrystals and promotes their dispersion in water but the stability commonly declines over time, forming aggregates that sediment and are difficult to redisperse ¹.

Regarding the preparation of CNF, mechanical defibrillation is applied to reduce the diameter of cellulose fibers, keeping longer nanofiber length; which involves high-pressure homogenization or microfluidization combined with chemical or enzymatic pretreatments, which reduced the energy consumption of the mechanical process. Due to the entanglement of long cellulosic chains, it is not so easy to determine the length of CNF, commonly higher than 1 μm, with microscopic techniques. CNF suspensions are distinguished from CNC by having higher aspect ratio, lower crystallinity index, higher water absorption capacity and lower light transmittance ¹¹. High nanofibrillation yield of CNF suspensions implies homogenization at consistencies between 1-3 wt% obtaining a stable gel structure which is difficult to disperse in separated nanofibers¹². The high viscosity of CNF suspension at low concentration makes very interesting as a non-caloric stabilizer and gellant in food applications but affect negatively when CNF is used as a reinforcement agent in a fiber network.

Several studies have evaluated the ability of CNF to improve mechanical paper properties in virgin ^13-15 and recycled pulps ^16-18. Moreover, some authors reported the relationship between poor dispersion of the CNF gel into the recycled pulp and lower tensile¹⁶, but extensive studies about nanocellulose dispersion are still needed.

Recently studies have demonstrated the feasible of flexographic ink removal using TEMPO-oxidized CNF hydrogels through its combination with cationic polyacrylamide (cPAM). The removal water-based ink mechanism is based on the adsorption of ink particles onto the CNF hydrogel followed by the flocculation of the system CNF-ink particles with cPAM addition¹⁹. However, the shear force generated during stirring had not been studied and could have an important impact in CNF gel structure and, therefore, in the removal ink mechanism.

Therefore, the dispersion of nanocellulose suspensions has rarely been investigated and continues to be a challenge for industrial implementation.

The objective of this study is to establish how the dispersion of the nanocellulose influence the final properties of the recycled paper and the water-based ink removal. On the one side, the effect of different pulping conditions and the use of dispersant agents were studied in presence of two retention additives. Results showed pulping conditions have an important impact on dispersion and retention of CNC and CNF, although it is dependent on the retention additive used. Moreover, the use of dispersant agent favored the dispersion of nanocellulose in the pulp, increasing tensile index of the recycled paper in 20.6% and 11.1%; using 1.5% CNF and 3.0% CNC, respectively. On the other hand, results proved that high stirring rate (750 rpm) during the interaction CNF-ink particles decreased the removal ink efficiency.

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