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Sample Softwood pulp samples SBKP Bleached kraft pulp SUKP Unbleached kraft pulp DSUKP Unbleached kraft pulp
Delignification treatment Klason lignin (%)
ECF-bleached
0.0 5.7 3.5
SKP
–
Delignified
CTMP Chemi-thermomechanical pulp –
37.9
Table 1. Klason lignin content of softwood pulp samples.
than the CTMP fiber. In addition, Yoshinaga et al. 27,28 devised a method of directly measuring the contact angle by continuously photographing the contact interface between the wood pulp fiber and the wet liquid using a stroboscope every 0.2 s. The dry and wet recycling treatment for pulp significantly increased the contact angle of water with respect to the pulp fibers, but tended to decrease the contact angle of water on the paper surface. In addition, Okayama et al. 29 evaluated the change in surface free energy generated in pulp fibers by recycling, including the contribution of acid–base interaction, and found that γ s hardly changed but γ s -decreased. Although many studies on paper recycling have been conducted over the years, it is well known that recy- cled pulp fibers reduce the strength of paper, mainly tensile strength, compared to virgin fibers. The effect of repeated wetting and drying treatments on the reduction of bond-forming ability between pulp fibers has been mainly due to the loss of the bond region caused by the hornification of fibers. Therefore, it has been considered important to improve the fiber flexibility in order to regain the bond-forming ability of fibers. However, it is not easy for the recovery of the tensile strength of paper prepared from recycled fibers by refining to reach the level of undried fibers 15 . This indicates that the recovery of the bond-forming ability of dried fibers does not reach that of undried fibers. Eastwood et al. 30 examined the effects of additional refining of pulp, stock preparation by adding sizing agents, and papermaking with hand-made or machine-made paper on the physical properties of paper for each recycling of semi-bleached kraft pulp. It was clarified that the recycling process by preparing the hand-made paper has a significant decrease in the tensile strength of the paper and greatly reduces the hemicellulose content in the paper as compared with the machine-made paper using a white water circulation system. As a result, it was concluded that the decrease in paper strength due to recycling is caused not only by the decrease in swelling capacity of secondary fibers but also by the deterioration of the fiber surface condition. Seth and Page 31 and Gurnagul et al. 32 argued that the decrease in the tensile strength of paper during drying is affected not only by the relative bond area but also by the decrease in shear bond strength from the examination by the Page’s equation. Therefore, in order to improve the strength characteristics of recycled paper, it is necessary to focus on increasing the bonding strength between fibers 32,33 . The wettability of the pulp fiber surface is an important factor for the fiber–fiber bonding force, and it is con- sidered that the improvement thereof increases the swelling property, flexibility and specific surface area of the fiber and leads to the strengthening of the fiber–fiber bonding force 34,35 . The wettability of the pulp fiber surface has a great influence on the swelling of the fiber and the liquid permeability of the paper, and the surface free energy of the pulp fiber can be calculated from the measurement of the contact angle of the liquid on the fiber surface. However, few studies have clarified the effect of the wettability of pulp fibers on the paper strength and fiber–fiber bonding of recycled fibers. In this study, in order to investigate the effect of the physicochemical properties of recycled pulp fibers on the paper strength, recycled pulps with different lignin contents prepared from softwood UKP and CTMP was examined from the change in the fiber–fiber bonding force based on the Page equation. Ǥ Commercially available softwood bleached kraft pulp (SBKP), softwood unbleached kraft pulp (SUKP) and chemi-thermomechanical pulp (CTMP) were used (Table 1). SUKP and SBKP were manufac- tured from mixed chips of Japanese larch, Douglas fir and slash pine, and CTMP was manufactured from Todo fir. Ǥ Chlorine dioxide bleaching treatment was performed on SUKP to prepare chlorine dioxide bleached softwood kraft pulp (DSUKP). For chlorine dioxide bleaching, 20 g of pulp was placed in a Lamizip. After 8 g of sodium chlorite powder was added, 4 ml of acetic acid were poured into the Lamizip together with 1200 ml of water. The zipper of the Lamizip was closed to prevent chlorine dioxide gas from escaping to the outside, and the Lamizip was placed in a hot water bath at 70–80 °C. After reacting for 1 h, 8 g of sodium chlorite and 4 ml of acetic acid were added. SUKP repeated this process twice. After completion of the reaction, the contents of the Lamizip were suction-filtered on a Büchner funnel, washed until acetic acid was exhausted, and finally washed with hot water. Lignin content was measured for all pulp samples according to TAPPI TEST METHOD T222. The results are shown in Table 1. Ǥ Pulp samples were beaten using a Valley beater according to ISO 5264- at a pulp concentration of 2% until a freshness of 400– 450 mL CSF. In order to prepare recycled pulps, handsheets were prepared from each beaten pulp sample based on ISO 5269-1. The handsheets were used as a sample (hereinafter abbreviated as R0) that was recycled 0 times. On the other hand, a part of the handsheets were dried at 80 °C for 24 h in a forced air circulation oven after
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