Cellulose nanofibrils (CNFs) produced by different mechanical methods to improve mechanical properties of recycled paper

•Relationship between CNF morphology and reinforced effect for recycled fibers.•CNFs had good reinforced effect and little damage on dewatering efficiency.•CNFs reinforced recycled fibers with no need of polyelectrolytes.•CNF form deposited on fibers and mechanism of reinforcement were discussed. In current study, CNFs produced by different mechanical methods, were used to improve the mechanical properties of recycled paper. The result showed the morphology of CNFs had great impact on reinforced effect and the length of fibrils determined their contribution in recycled paper strength. For different CNFs with similar diameter, the higher aspect ratio resulted in better reinforced effect. The CNFs produced by microfluidic homogenization and suitable PFI milling conditions (RM-CNF1) got best reinforced effect which improved tensile index and burst index by 35.5 % and 49.4 % at 5.0 wt% addition, respectively, due to their high aspect ratio. Although the CNFs produced by ball milling and ultrasonication (BU-CNF2) still had many bundles that were not fibrillated completely, their reinforced effect just below RM-CNF1 due to their special morphology and high retention rate. This work aims to study the influence of CNFs on recycled fibers reinforcement.