Appl. Sci. 2025 , 15 , 9036
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Figure 7 provides optical microscopy images of KOCC fibers prior to and following the application of two different refiner plate types: a conventional cast plate (CEL 37 km/s) and an ultra-fine bar plate (CEL 97 km/s). The disintegrated fibers before refining in Figure 7a appear as elongated, uncut fibers with minimal fines or debris. Most fibers remain bundled, indicating that only limited morphological change has occurred due to initial disintegration. Refining with the conventional cast plate, shown in Figure 7b, results in evident morphological alterations. The fibers exhibit substantial external fibrillation and increased formation of short fiber fragments and fines. Many fibers are noticeably shortened or cut, and the fiber network appears heterogeneous with a greater presence of debris among the fibers. Such modifications are indicative of an aggressive refining action that enhances fiber separation and increases the surface area for bonding but may also introduce an excess of fines. In contrast, the use of the ultra-fine bar plate in Figure 7c yields a different fiber morphology. The resulting fibers largely retain their original length and structure, with fewer short fibers and fines produced. Fibrillation is present but more uniform and less pronounced. The overall structure is coherent with fewer visible debris particles, reflecting the more controlled action of the ultra-fine bar plate. These differences highlight the important role of plate selection in controlling fiber breakdown, fines generation, and network uniformity [5,10,11]. Figure 8 shows the corresponding differences in sheet structure as observed by SEM. Sheets formed from disintegrated fibers (Figure 8a) reveal loosely bound networks with smooth fiber surfaces and limited bonding. When fibers refined at CEL 37 km/s are used (Figure 8b), the resulting sheet displays a denser and more compact network, characterized by abundant fines that fill void spaces and improved interfiber bonding, though often at the expense of drainage and formation due to the excessive presence of fines. Sheets derived from fibers refined at CEL 97 km/s (Figure 8c) show a well-developed network structure with strong bonding and relatively fewer fines. The sheet retains an open yet interconnected architecture, consistent with the more moderate morphological changes induced under these conditions. Together, the optical and SEM analyses suggest that the severity and type of refining have a direct influence on both fiber and sheet morphologies. Refining with a conventional plate promotes fragmentation and fines production, while refining with an ultra-fine bar plate better preserves fiber length and integrity, resulting in improved fiber network formation. These morphological distinctions are important in the context of optimizing pulp quality and papermaking performance. 4. Conclusions This study demonstrates that refining KOCCs with an ultra-fine bar plate at a CEL of 97 km/s significantly improves fiber and paper properties compared to a conventional plate at 37 km/s. While unrefined KOCCs exhibited limited strength and bonding, the use of the ultra-fine bar plate led to marked enhancements in tensile strength, tear strength, water retention value (WRV), and fiber bonding, even though a reduction in paper bulk was observed. The superior design of the ultra-fine bar plate promotes more effective fibrillation, resulting in a denser and more cohesive fiber network at lower energy inputs. These findings highlight the ultra-fine bar plate’s effectiveness in upgrading recycled KOCC fibers, providing a promising strategy for producing high-performance recycled paper that meets demanding industry standards.
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