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3.4. Water Retention Value Figure 4 illustrates how the water retention value (WRV) of Korean Old Corrugated Containers (KOCCs) fibers evolves when refined with two different refiner plates, featuring cutting edge lengths (CELs) of 37 km/s and 97 km/s. The results show that as specific refining energy increases, WRV also slightly increases for both refiner plates. This trend supports the well-established understanding that higher refining energy enhances the water retention capacity of fibers, which is beneficial for improving fiber bonding and the mechanical strength of paper [28,29].
Figure 4. Comparative Analysis of WRV versus specific refining energy (kWh/t) for KOCCs at CELs of 37 km/s and 97 km/s. The ultra-fine bar plate at CEL 97 km/s is particularly effective at promoting both external and internal fibrillation, resulting in slightly higher WRV at equivalent refining energy levels compared to the cast plate at CEL 37 km/s. This finding is consistent with previous research, which reported that refiner plates with sharper bar edges and narrower grooves induce more effective fibrillation, thereby increasing the fiber surface area and water retention [5,7]. Enhanced fibrillation not only improves water retention value (WRV) by increasing the fiber’s surface area and swelling capacity but also strengthens the bonding potential of recycled fibers through the formation of more fibrillar bonds, which is especially important for KOCCs that have undergone multiple recycling cycles [27,30]. WRV is a critical parameter for assessing the effects of refining on fiber properties. Higher WRVs indicate improved water-holding capacity, which is essential for enhancing fiber bonding and the mechanical properties of the final paper product [24,29]. WRV is influenced by both external and internal fibrillation, as well as the content of fines [26]. For recycled fibers such as KOCCs, optimizing WRV through refining is crucial for significantly improving paper quality and performance [31]. The observed increase in WRV with higher specific refining energy for both refiner plates underscores the importance of refining energy in enhancing KOCC fiber water reten- tion. However, the ultra-fine bar plate at CEL 97 km/s demonstrates a more pronounced improvement in WRV, attributable to its superior capacity for inducing fibrillation [5,7]. This makes the ultra-fine bar plate particularly suitable for ultra-low intensity refining of recycled fibers like KOCCs. The ultra-fine bar plate’s design, with narrower grooves and sharper bars, intensifies fibrillation by applying more focused shear and compression forces to the fibers. This enhanced fibrillation increases fiber swelling and creates more fibrillar surfaces, which improves WRV—a key factor for stronger fiber bonding and paper strength. Although the narrower grooves can reduce stock throughput due to higher flow resistance, the plate optimizes fiber-bar contact to modify fibers efficiently with minimal excessive cutting. As
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