www.nature.com/scientificreports/
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Figure 5. Effect of recycling on water contact angle of DSUKP fibers with time variations.
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Figure 6. Effect of recycling on water contact angle of CTMP fibers with time variations.
(R1). The contact angle of water showed a tendency to gradually decrease as the number of recycling increased after 4 s of contact. This can be related to the appearance of radial cracks in the S2 layer of the fiber cell wall confirmed by TEM observation, when the SBKP fiber is recycled 3 times or more 8,9,15 . That is, the reason why the contact angle of water on the surface of SBKP fiber recycled 3 times or more gradually decreases is that as the contact time between the fiber surface and water becomes longer, water is penetrated into the cracks generated in the fiber cell wall of the recycled pulp. As a result, it is estimated that the water contact angle has decreased. As with the SBKP, the contact angle of water with respect to the SUKP fibers increases as the number of recycling increases (Fig. 4). The contact angle of the SUKP fiber after 1 s contact with water was 35°, which was slightly higher than that of the SBKP fiber. The SUKP fiber contains 5.7% of lignin (Table 1), and the change in the contact angle of water is influenced by the residual hydrophobic lignin. It would be caused that the surface of cellulosic fibers becomes hydrophobic again due to the redistribution of olefinic substances derived from wood materials, in addition to hornification of pulp fibers and cleavage due to hydrolysis of covalent bonds of cellulose chains during recycling 38 . In unbleached chemical pulp and mechanical pulp, lignin remains in the pulp, and the presence of triglyceride fat, fatty acid, resin acid and unsaponifiable matter has been confirmed, and it is possible to defibrate pulp fibers and heat-dry paper in the papermaking process. It is said that storage causes a self-sizing phenomenon in which these low-surface free-energy substances migrate and spread to the surface of pulp fibers 34,39,40 . Hodgson et al. 25 determined the water contact angle of pulp fibers using the Wilhelmy method after heat- treating Douglas fir kraft pulp at 105 °C for 16 h, and found that this heat treatment increased the contact angle. Therefore, it is shown that the wettability of the pulp fiber is suppressed by the self-sizing effect. Therefore, the contact angle of the SUKP pulp fibers after the first recycling increased to 41°, and after third recycling, the contact angle increased to 45°, showing a tendency for the contact angle to increase with the num- ber of recycling. It is considered that this is due to the effect of self-sizing. In addition, the contact angle of water with respect to the SUKP fiber after three recycling treatments was larger than that of the SBKP regardless of the passage of time. When the SUKP was delignified (DSUKP), the recycling treatment increased the contact angle of the pulp fibers (Fig. 5). The contact angle of the DSUKP pulp fiber before recycling is 30°, which is lower than that of the SUKP. The lignin content of the DSUKP pulp fiber was 3.5%, when the SUKP was delignified. It is considered that as the hydrophobicity of the pulp fiber was reduced, the contact angle was lowered. In addition, it was confirmed that the contact angle of the pulp fiber increased to 41° after the first recycling treatment and 43° after three recycling treatments, but it was somewhat lower than the SUKP.
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(2022) 12:1560 |
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