Appl. Sci. 2025 , 15 , 9036
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in mean fiber length. The improved fiber-fiber bonding, supported by higher WRV and a denser fiber network, offsets the negative effects generally associated with shorter fibers and higher fines content. This synergy aligns with previous findings [5,7,28], confirming that enhanced fibrillation and bonding quality can play a decisive role in sustaining tear strength in recycled KOCC papers, even as average fiber length declines. Balancing tensile strength and bulk is essential, depending on the end-use require- ments of the paperboard. Higher tensile strength improves durability and resistance to tearing, making it suitable for packaging applications requiring enhanced mechanical performance, such as shipping containers and industrial cartons. In contrast, higher bulk contributes to stiffness and a softer feel, which are desirable qualities for paperboard used in consumer packaging and folding cartons. If high tensile and tear strength are priori- tized, the ultra-fine bar plate (CEL 97 km/s) at lower refining energy is recommended, as it efficiently enhances fiber bonding and preserves fiber length. However, for paper- board products where greater bulk or a softer feel is required, the conventional plate (CEL 37 km/s) may be more appropriate. 3.5.3. Fiber Morphology Analysis Figures 7 and 8 present a comparative analysis of KOCC fiber and sheet morphologies before and after refining, as observed through optical and scanning electron microscopy (SEM), respectively. All samples were refined to a consistent Canadian Standard Freeness (CSF) of 200 mL to ensure valid comparisons across different refining conditions.
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( b ) ( c ) Figure7. Optical microscopy images showing KOCC fiber morphology before and after refining at 37 km/s and 97 km/s plates (all samples refined to 200 mL CSF). ( a ) KOCC fiber morphology before refining, ( b ) after refining with a 37 km/s plate. ( c ) after refining with a 97 km/s plate.
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( b ) ( c ) Figure8. SEM micrographs of KOCC sheets prepared from fibers before and after refining at 37 km/s and 97 km/s plates (all samples refined to 200 mL CSF). ( a ) KOCC fiber morphology before refining, ( b ) after refining with a 37 km/s plate. ( c ) after refining with a 97 km/s plate.
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