Development of High-Loading Filler for High Bulk Paper Using Nanocellulose Prepared by Ca(OH)2 Assisted Fibrillation

In the papermaking industry, simultaneously achieving high filler loading and high paper bulk remains a critical challenge for lightweight-paper production and resource conservation.However, conventional inorganic fillers typically increase sheet density, limiting bulk improvement and causing significant deterioration in mechanical strength.Herein, cellulose nanofibrils (CNFs) were prepared using an energy-efficient Ca(OH) 2 -assisted mechanical fibrillation process and directly utilized as structural scaffolds for the in situ synthesis of CaCO 3 -CNF composites known as flexible calcium carbonate (FCC).Ca(OH) 2 -assisted treatment induced alkaline swelling and structural relaxation of pulp fibers while increasing particle-fiber friction during grinding.This synergistic effect enabled the production of CNFs with an average width comparable to that obtained for untreated pulp after five grinding passes but using only a single grinding pass.At a CNF width of 60-65 nm, the sample treated with 0.5 wt% Ca(OH) 2 and subjected to one grinding pass required ~24% less specific grinding energy than the untreated five-pass control.Subsequently, FCC fillers were synthesized through carbonation, obtaining unique composite aggregates wherein precipitated calcium carbonate was directly integrated into the fibril network.When applied to handsheets, the FCC fillers effectively preserved fiber-to-fiber bonding owing to their large effective particle size and high aspect ratio, considerably increasing sheet bulk while mitigating the reduction in the breaking length observed generally.