Appl. Sci. 2025 , 15 , 9160
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( a ) Disintegrated HwBKP
( b ) Beaten HwBKP
( c ) Re ę ned HwBKP
( d ) Disintegrated SwBKP
( e ) Beaten SwBKP
( f ) Re ę ned SwBKP
Figure 12. Optical micrographs showing morphological changes in HwBKP and SwBKP at ap- proximately 200 mL CSF: ( a ) Disintegrated HwBKP, ( b ) Beaten HwBKP, ( c ) Refined HwBKP, ( d ) Disintegrated SwBKP, ( e ) Beaten SwBKP, ( f ) Refined SwBKP. 4. Conclusions This study demonstrated that ultra-fine bar plate refining offers substantial advantages over Valley beating for both softwood (SwBKP) and hardwood (HwBKP) bleached kraft pulps. The disc refiner with an ultra-fine bar plate rapidly reduced pulp freeness from ~700 mL to ~200 mL CSF within approximately 10 min, while Valley beating required over 60 min to reach comparable freeness. At similar freeness levels (~200–300 mL CSF), tensile strength for refined SwBKP improved by about 35%, and HwBKP by approximately 20%, compared to beaten fibers. Burst and tear strengths also increased significantly; for instance, the tear strength of refined SwBKP was about 25% higher than after beating. Optical microscopy confirmed that refining effectively enhances fibrillation and fiber bonding while better preserving fiber length than beating, resulting in improved mechanical properties of paper sheets. These quantitative findings underscore the enhanced efficiency and performance of ultra-fine bar refining in pulp treatment and highlight its importance for optimizing paper strength and quality in sustainable papermaking processes. Author Contributions: Research and investigation, C.-H.K., M.-S.P., H.-H.P., J.-H.P. and J.-S.L.; tables and figures preparation, J.-H.P. and J.-S.L.; data curation, H.-H.P. and M.-S.P.; writing—review and editing, H.-H.P. and C.-H.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Basic Science Research Program via the National Re- search Foundation of Korea (NRF), funded by the Ministry of Education (Grant No. RS-2022- NR07562261382116530004). It was also supported by the Korea Basic Science Institute (National
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