Papers with high filler contents enabled by nanocelluloses as retention and strengthening agents.

The development of economic and functional papers relies on the introduction of functional filler particles but faces several challenges, especially the low filler retention and inferior mechanical properties. This study introduces plant-based cellulose nanofibrils (CNFs) as dual-function retention and strengthening agents, offering a sustainable alternative to petroleum-based polymer additives. Precipitated calcium carbonate (PCC), a widely used filler, is selected as the model system. The effects of CNF dosage, aspect ratio, and surface carboxyl content on the filler flocculation process were systematically investigated, with additional enhancement achieved through cationic ion crosslinking to improve filler bridging. Remarkably, a superior filler retention rate (>90 %) and filler-tensile factor (2.0) were achieved for the final papers at very low CNF dosage (1 %), surpassing most literature data. Detailed analyses of filler morphology and distribution elucidated the structure-property relationships underlying this performance. Furthermore, the versatility of the approach was demonstrated with other functional fillers, imparting properties such as flame retardancy, oil/water separation, high brightness, and soluble organic pollutant absorption. Overall, this study reveals the critical feature of CNFs in simultaneously improving filler retention and mechanical properties, demonstrating their high potential for their applications in the papermaking industry.