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a result, the ultra-fine bar plate achieves superior WRV improvement and fiber quality while maintaining or reducing energy consumption compared to conventional plates with wider grooves. In conclusion, the ultra-fine bar plate at CEL 97 km/s is optimal for improving the properties of KOCCs in papermaking, providing enhanced water retention and fiber bond- ing through effective fibrillation. Balancing refining energy and the choice of refiner plate is essential for optimizing the refining process and achieving superior paper quality, as reflected in improved mechanical strength (tensile, burst, and tear strength), fiber length retention, and sheet uniformity [5,27,28].
3.5. Physical Properties 3.5.1. Bulk
A comparison of the bulk properties of paper made from KOCCs refined with 37 km/s and 97 km/s CEL plates is provided in Figure 5. Paper bulk, which reflects the volume occupied by a given mass of paper, is a key property influencing texture, porosity, and end-use performance [29,32].
Figure5. Comparison of paper bulk versus specific refining energy for KOCCs at CELs of 37 km/s and 97 km/s. The results show that as specific refining energy increases, paper bulk decreases for both refiner plates. This trend is consistent with established findings that higher refining energy compacts the fiber network, leading to denser and less bulky paper [28,33]. Notably, the ultra-fine bar plate with a CEL of 97 km/s produces paper with slightly lower bulk compared to the plate with a CEL of 37 km/s at equivalent energy levels. This outcome suggests that the ultra-fine bar plate, which promotes more extensive external and internal fibrillation, results in a tighter, more compact fiber structure [5,7] Paper bulk is a crucial parameter in determining the suitability of paper for various end-use applications. Higher bulk is generally associated with increased thickness and porosity, which contribute to a softer tactile feel and greater liquid absorbency—properties particularly desirable in tissue papers and the middle ply of cartonboard used for cush- ioning and insulation [30]. In contrast, lower bulk results in denser, smoother sheets with enhanced mechanical strength, making them better suited for printing and writing papers where surface uniformity and durability are essential [33,34]. The observed decrease in bulk with increasing refining energy underscores the im- portance of refining conditions in tailoring paper properties from KOCCs. The ultra-fine bar plate at 97 km/s is particularly suitable for applications requiring a lower bulk and a higher density, as it increases fiber bonding and network densification. Conversely, for products where a higher bulk is beneficial—such as tissue, copy paper, or certain packaging
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