Intact and broken cellulose nanocrystals as model nanoparticles to promote dewatering and fine-particle retention during papermaking

Cellulose nanocrystals (CNCs), either in intact form or after mechanical shortening, were used as a model nanoparticle for enhancement of dewatering and fine-particle retention during lab-scale papermaking process evaluations. Cryo-crushing, using dry or wet CNCs, was performed to shorten the particles from an initial mean value of 103.1 nm to either 80.4 nm (wet crushed) or 63.4 nm (dry crushed). Papermaking-related tests were performed with the solids from 100 % recycled copy paper, which were prepared as a 0.5 % solids suspension in dilute Na2SO4 solution and then treated successively with 0.05 % of poly-diallyldimethylammonium chloride, 0.05 % of very-high-mass cationic acrylamide copolymer, and then various types and dosages of negatively charged nanoparticles. The performance of the CNCs, relative to papermaking goals, was compared to that of two colloidal silica products that are widely used in industry for this purpose. All of the nanoparticles were observed to promote both dewatering and fine-particle retention. The intact CNCs were more effective than the broken CNCs with respect to fine-particle retention. Effects on flocculation of the fiber suspension were detectable, but not large relative to the sensitivity of the test employed. Results are discussed in the light of concepts of polyelectrolyte bridges and the participation of elongated nanoparticles in completing those bridges in such a way as to form shear-sensitive attachments among solids surfaces in the suspension.