www.nature.com/scientificreports/
Figure 7. Effect of recycling on contact angle of pulp fibers immersed in water after contact 1 s.
SBKP
SUKP CTMP
DSUKP
80
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-1
0
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Figure 8. Effect of recycling on tensile strength of handsheets prepared from various pulps.
In the case of the CTMP, the contact angle of pulp fibers tended to decrease after the recycling treatment (Fig. 6), showing a tendency different from that of softwood pulp fibers (SKP) whose water contact angle increased due to recycling. The contact angle before the recycling treatment was 48°, and the contact angle decreased to 43° after the first recycling treatment and 38° after three recycling treatments. It is presumed that the decrease in contact angle with the increase in the number of recycling is due to the decrease in the content of lignin and wood-derived resin during the recycling process. A slight increase in the tensile strength of the CTMP sheet due to the recycling process (Fig. 8) also supports this possibility. The contact angle of CTMP is higher than that of softwood pulp fiber, and it can be judged that the effect of hydrophobicity of lignin and resin increases the contact angle with water 21 . Figure 7 shows the effect of recycling treatment on the contact angle of pulp fibers at 1 s after contact with water. It was clarified that the contact angle of SBKP and SUKP fibers tends to increase as the number of recycling treatments increases, but the contact angle of CTMP fibers decreases as the number of recycling increases. ơ Ǥ Figure 8 shows the changes in the tensile strength of handsheets prepared from each pulp sample by recycling. As the number of recycling treatments increased, the tensile strength of the handsheets prepared from the SKP pulps tended to decrease. In the case of the SBKP, the tensile index of handsheets gradually decreased from 65.9 N m/g in R0 to 55.7 N m/g in R1 and 53.8 N m/g in R3 due to the recycling process. There was a tendency that the tensile index of the SUKP handsheets is lower than that of the SBKP, which decreases from 63.4 N m/g in R0 to 54.5 N m/g in R1 and 48.0 N m/g in R3 due to recycling. It was revealed that delignification (DSUKP) treatment using sodium chlorite and acetic acid improves the tensile strength of handsheets before recycling. In addition, the tensile strength of DSUKP handsheets was higher than that of the SUKP regardless of the presence or absence of recy- cling treatment. Since the presence of lignin interferes with the formation of interfiber bonds in the handsheets, the delignification treatment could improve the tensile strength of the handsheets. The tensile index of DSUKP handsheets decreased from 71.2 N m/g before recycling (R0) to 65.4 N m/g for R1 and 61.1 N m/g for R3 as the number of recycling increased. On the other hand, in the case of the CTMP in which components other than cellulose such as lignin remain, the tensile index of handsheets increases from 28.6 N m/g in R0 to 32.2 N m/g in R1 and 36.8 N m/g in R3 as the number of recycling increases. R3 showed a tendency to increase slightly to 36.8 N m/g. This is partly due to the fact that in experiments conducted by Howard et al. 18 using groundwood pulp (GP) and CTMP,
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(2022) 12:1560 |
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