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Cellulose (2020) 27:7215–7225
stiffness index was observed, and a maximum increase of 27% was noted with the CNF treated with DES and the highest grinding level (T3). Furthermore, the most economical way to increase the tensile stiffness was to add the CNF with a moderate grinding level (i.e., U2, Fig. S5). For example, the 6 wt% dosage of T2 resulted in a tensile stiffness index of approximately 5.5 MNm/kg with a relative cost of 160%, while cost related to the use of the 6 wt% of U3 was approxi- mately 190% with the stiffness index of approximately 5.3 MNm/kg. In the best case, the tensile stress (z-direction tensile strength) was 85% higher (i.e., T3 [6%]) than that of the reference board handsheet (i.e., without grinding) (Fig. 7). However, the differences between the DES-treated CNF and the reference CNF were small. Similar to the tensile strength and stiffness values, the most cost-efficient use of reinforcement was to add the CNF with the moderate grinding levels (i.e., U2) to enhance the z-direction tensile strength (Fig. S6). Overall, the results clearly demonstrate that the best way to improve the mechanical properties of the board sheet was to add approximately 4 wt% of untreated mechanical grinded CNF from the moderate grinding levels (those that used grinding energy 3–4 kWh/kg) as a reinforcement material. Doing so only increased the relative price of the manufactured board by 15–20%, but the tensile strength index improved 25–40%, the tensile stiffness index was 10–20%, and the z-direction tensile strength was 40–60% (Fig. S4–
Fig. 5 Tensile strength index against reinforcement fiber amount of the prepared board handsheets
pretreatment, a more economical way to enhance the strength properties would be to add untreated mechan- ical grinded CNF as the reinforcement. Of the CNF used, the samples obtained from moderate grinding (U2) resulted in the most cost-efficient board sheets (Fig. S4). The obvious reason for this phenomenon is that the energy consumption of grinding increased significantly at the end of the grinding process (i.e., U2 ? U3), while the improvement in strength prop- erties was not as pronounced. The trend for the tensile stiffness index of the board sheets was similar to the trend for the tensile strength index (Fig. 6). A nearly linear increase in the tensile
Fig. 6 Comparison of the tensile stiffness index based on the amount of reinforcement fiber of the prepared board handsheets
Fig. 7 The tensile stress (z-direction tensile strength) rein- forcement fiber amount of the prepared board handsheets
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