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Figure 1. Schematic diagram of measuring water contact angle between single pulp fiber and water.
the pressing. Further, after immersing the dried handsheets into water for 1 h, the handsheet was sufficiently defibrated by a disintegrator to prepare the handsheet again. This was used as a sample for one recycle (herein- after abbreviated as R1). In this procedure, wetting and drying were repeated up to 3 times to prepare recycled pulp handsheet samples (hereinafter abbreviated as R3). The prepared test handsheet was adjusted for humidity all day and night under environmental conditions of 23 °C and 50% RH, and then the sheet density and tensile strength were measured according to ISO 5270, and the zero-span tensile strength was measured according to ISO 15361. ƤǤ The optical technique to measure directly the contact angle of a water drop against a single pulp fiber was used for characterizing fiber-liquid interactions. A system consisting of a single-lens reflex camera (Nikon D3), a 24 mm wide-angle lens (Nikon), a bellows attach- ment (Nikon MD-4), an extension bellows attachment (Nikon BP-6E) was used for the measurement (Fig. 1). A xyz stage (Chuo Precision Industrial) was also used to hold a pulp fiber. The 24 mm wide-angle lens mounted to the bellows attachment was attached to the camera. The extension bellows attachment connected in series to increase the shooting magnification. The handsheet was carefully torn so as not to break the inter-fiber bonds in the paper sheet, and the surround- ing fibers were excised, leaving one pulp fiber in the torn handsheet cross-section 36 . Pulp fibers were selected from relatively straight samples and measured 20 times per sample. After fixing the tip of the pulp fiber downward, the height of the tip of the pulp fiber was made constant. Pure water was put onto a slide glass in which a liquid reservoir was placed on the z-direction lift, and the z-direction lift was raised to make the tip of the pulp fiber pure water. The altitude at which the z-direction lift was raised was also constant, and the distance at which the pulp fibers were inserted into the water was the same 27,29 . After the tip of the pulp fiber came into contact with water, the area around the pulp fiber was continuously magnified and photographed every second for 10 s or more (Fig. 2). The P values between all of the data were all 0.05 or less, thus indicating a significant difference. The starting point of measurement (0 s) was defined as the point where fiber insertion could be confirmed from the continuously captured images. When the pulp fiber is in contact with water, the contact surface is quickly wetted and absorbed water. At this time, the contact angle changes to a certain extent, until the contact angle becomes stable when it is saturated (Figs. 3, 4, 5, 6). ơ ƤǤ Figures 3, 4, 5 and 6 show the changes over time in the contact angle of water in various recycled pulp fibers. In the case of SBKP, the contact angle increased with the number of recycling increased (Fig. 3). The contact angle at 1 s after water comes into contact with the pulp fiber was 32° for the pulp fiber (R0) before recycling, while it was 44° after the first recy- cling (R1). However, the contact angle was 44° even after 3 times of recycling (R3). There was little difference in the contact angle at 1–3 s after the contact, even if the number of recycling treatments increased. Therefore, it was suggested that the first recycling treatment out of the three recycling treatments had a particularly large effect on the wettability of pulp fibers. The increase in contact angle due to the recycling process of pulp fiber can be caused by both the hornification of the pulp fiber surface shown in the decrease in pore volume in the fiber cell wall 12,15 , and the decrease in surface bonding potential 30,32,33 . On the other hand, Wistara et al. 37 measured the contact angle of water using the Wilhelmy method to evalu- ate the surface properties of recycled pulp fibers, and found that the recycling treatment reduced the contact angle of bleached kraft pulp fibers. Regarding the change in wettability on the pulp fiber surface due to recycling, our results tended to differ from their results because of the difference in the treatments of the recycling process. That is, in the test of Wistara et al., the pulp was beaten after being repeatedly defibrated in water, dewatered and dried in the recycling process, but in our experiment, the fact that the pulp was not beaten after the recycling process
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(2022) 12:1560 |
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