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layers—the conventional plate at CEL 37 km/s may be more appropriate, as it allows for greater paper bulk and the associated functional advantages [5,28]. In summary, optimizing the choice of refiner plate and refining energy enables the pro- duction of KOCC-based papers with properties tailored to specific end-use requirements. 3.5.2. Tensile and Tear Strength The tensile strength results for KOCCs refined using two different CEL refiner plates are shown in Figure 6a. For both plates, tensile strength increases as specific refining energy rises, demonstrating a positive correlation between refining energy and fiber bonding. The ultra-fine bar plate (CEL 97 km/s) consistently achieves higher tensile strength than the conventional plate (CEL 37 km/s) at equivalent energy levels, and the rate of increase is steeper. This indicates that the ultra-fine bar plate is more efficient in enhancing fiber bonding and tensile strength with lower energy input. These results are supported by pre- vious research, which found that refining plate geometry and bar parameters significantly influence fiber modification and strength development during low-consistency refining [34]. Enhanced fibrillation increases the surface area for hydrogen bonding between fibers, a primary mechanism for tensile strength improvement in recycled pulps [5,7].
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( b )
Figure6. Comparison of strength properties versus specific refining energy for KOCCs at CELs of 37 km/s and 97 km/s; ( a ) Tensile strength; ( b ) Tear strength. Figure 6b examines the tear strength of KOCCs refined using the same plates. For CEL at 37 km/s, tear strength shows a slight decline as refining energy increases, starting at 724 mN and dropping marginally at higher energy levels. In contrast, tear strength for CEL at 97 km/s remains stable, ranging from 724 to 743 mN across the energy spectrum, and is generally higher than that achieved with the conventional plate. This stability suggests that the ultra-fine bar plate is more effective at preserving fiber length during refining, which is critical for maintaining tear resistance [7,27,35]. The relationship between refining, fiber length, and tear strength is well documented in previous studies: excessive refining or aggressive plate patterns can significantly reduce fiber length and consequently tear strength, whereas optimized refining conditions can effectively balance fiber bonding and fiber length retention to maintain or enhance tear resistance [28,32]. The ability of the ultra-fine bar plate to maintain or slightly improve tear strength, despite promoting faster fiber shortening (as seen in Figure 3a), is explained by its impact on overall fiber network structure. While the ultra-fine plate increases fines content and fiber cutting, it also greatly enhances external fibrillation due to the concentrated mechanical action of its narrower grooves and sharper bars. This results in a more extensive fibrillar surface area, which substantially strengthens inter-fiber bonding. As a result, the network integrity and sheet toughness are maintained or even improved, compensating for the reduction
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