Nanomaterials 2022 , 12 , 790
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Figure 8. Mechanical properties of handsheets prepared with CNF and OCC stirred together at different speeds: ( a ) Porosity; ( b ) Bulk; ( c ) Tensile Strength Index; ( d ) Bursting Index; ( e ) SCT Index; ( f ) Tear Index. Moving to mechanical properties, tensile strength is studied in Figure 7c. OCC blank has a tensile index of 33.1 kN · m/kg. When CNFs are stirred at several speeds, the tensile index increases progressively with the stirring speed until 40.7 kN · m/kg, when CNFs are stirred at a velocity gradient of 500 s − 1 , in which the Ø g is minimal with the highest AR. It is produced an increase of 23% respect to blank and 18% compared with the sample with CNFs stirred at the low speed. This fact is due to the progressively separation of the CNF clusters from the gel with the agitation, with a more opening CNF network and better homogeneity in the suspensions and then a better distribution of the fibers in the final matrix, increasing the strength. However, an excess on the velocity gradient, above the minimumØ g (3000 s − 1 ), does not improve the results with a tensile index of 37.7 kN · m/kg and an increase of 13.7% respect to blank. This fact may be due to the break of the fibers in
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