# Study of Refining Effect of Mixed Pulps Using Refiner Plates with Different Bar Patterns
## 3 Results and Discussion
### Physical Properties of Paper  
It is necessary to refine or beat pulp fibers to develop appropriate fiber properties for papermaking. After refining, pulp fibers are flattened and become more flexible, and have a higher area available for bonding [25]. Figure 10 shows the effect of CEL on the bulk of the sheets made from the mixed pulps. In general, the paper bulk was negatively affected by refining irrespective of pulp mixing ratios or plate CEL. As Sw-BKP was mixed with Hw-BKP, more refining energy was consumed to achieve the same bulk. In particular, PB97, with larger CEL, consumed about half of the refining energy compared to PA55 to reach the same bulk at each mixing ratio.  
Figure 10: Effect of CEL on bulk of paper made of the mixed pulps: (**a**) PA55; (**b**) PB97.  
Figure 9: Effect of CEL on WRV of the mixed pulps: (**a**) PA55; (**b**) PB97.  
It was confirmed that, when refining the mixed pulp with a fine bar plate with a very large CEL, paper with good bulk could be produced while consuming less refining energy.  
Figure 11 compares the effect of CEL on the tensile strength of the sheets made from mixed pulps. Irrespective of the CEL difference, the refining effects for Sw-BKP were slowly developed, but it had the greatest tensile strength at the target freeness. On the other hand, the refining effects for Hw-BKP were rapidly developed, but the tensile strength at the target freeness was the lowest.  
In the case of PA55, the refining energy for the mixed pulps of Hw-BKP and Sw-BKP required up to the same tensile strength was less than the energy for the independent refining of Sw-BKP and Hw-BKP. PB97 also showed a similar trend, but the refining energy required for the same tensile strength was less than that of PA55. The tensile strength in the target freeness was greater in PB97 than in PA55, irrespective of the mixing ratios of Sw-BKP and Hw-BKP. As more Sw-BKP was mixed, greater refining energy in PB97 was required at the same tensile strength [3]. It was finally confirmed that the fine bar plate with a very large CEL was highly advantageous in strength development and energy saving.  
Figure 12 compares the effect of CEL on the tear strength of paper prepared from the mixed pulps of Sw-BKP and Hw-BKP. It is known that tear resistance is closely related to the length of the pulp fibers. Longer fibers prepare weakly bonded sheets, but the tear strength of these papers is improved. Refining contributes to the production of well-bonded sheets, but improved bonding strength reduces the tearing resistance [26]. PA55 and PB97 plates with sharp bar edges improved tear strength irrespective of the pulp mixing ratios during refining. However, PA55 had better tear resistance as the mixing percentage of Sw-BKP increased. In particular, when Sw-BKP and Hw-BKP were mixed at a ratio of 7:3, it showed much better tear resistance than Sw-BKP under less refining energy. These mixed pulps were generally similar to the tear resistance behavior of typical softwood pulp. The mixing of short fibers with long fibers seemed to help improve interfiber bonding [27].  
Figure 11: Effect of CEL on tensile strength of paper made of mixed pulps: (**a**) PA55; (**b**) PB97.  
Figure 12: Effect of CEL on tear strength of paper made of mixed pulps: (**a**) PA55; (**b**) PB97.  
Unlike PA55, when refining with PB97, tear resistance increased irrespective of the pulp mixing ratios of Sw-BKP and Hw-BKP, and the sheets prepared from the pulps with more Sw-BKP showed better tear resistance than those prepared from Hw-BKP alone at each refining step. It was interesting to note that the sheet with a mixing ratio of Sw-BKP and Hw-BKP of 7:3 also displayed the best tear resistance. In particular, much less refining energy in the final tear strength was much less consumed in the fine bar plate PB97 than in PA55.