7220
Cellulose (2020) 27:7215–7225
Fig. 1 Surface structure of the board handsheets with the absence and presence of CNF as reinforcements; a board from 100% of T0; b board containing 6 wt% of T1; c board containing 6 wt% of T2; d board containing 6 wt% of T3
preparation of the handsheets, the drainage time was observed to increase almost linearly (Fig. 2) as a function of the reinforced CNF dosage, which varied from 2 to 6 wt%. Moreover, the increase in the grinding time of the fibers decreased the water removal, i.e., it increased the drainage time (U1 ? U2 ? U3). A similar trend was observed for the DES- treated and reference CNF. However, the total reten- tion of the board handsheets were very high (from approx. 97.5–99.5%), which was in practice 97.5%
with the highest dosage of reinforcement CNF (6 wt%) and with CNF of the smallest size (U3 and T3). This indicates that the retention of the reinforce- ment CNF was also high without the use of any retention chemicals (around 70%). The reinforcement of the mechanical properties of the board sheets by the addition of CNF is most likely connected to the enhanced bonding between the board fibers promoted by the CNF (da Costa Correia et al. 2016). Therefore, CNF can increase the density and stiffness of a sheet, and decrease the sheet bulk. In the present study, the highest density (Fig. 3) and lowest bulk (Fig. S3) were observed, as expected, in the board sheets with the highest amount (6 wt%) of the most fibrillated reinforcement CNF (U3). For example, the sheet density increased from 577 to 684 kg/m 3 , while bulk decreased from 1.73 to 1.46 cm 3 /g with U3. Density trends and therefore in turns bulk trends seems to be similar in both for the DES-treated and reference CNF. The tear index and strain of the board handsheets are presented as a function of the CNF reinforcement (Fig. 4.) Both the tear index and strain increased with larger CNF dosages, except for the sheet containing U1. However, the effect of CNF on the tear strength was either relatively small or it plateaued at low dosages. Furthermore, the improvement in strain
Fig. 2 Drainage time of the board handsheets
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