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in the resulting pulp properties [24,29]. While the Valley beater provides valuable initial insights into pulp fiber response to mechanical treatment, it may not capture the nuanced effects observed in refining, especially for pulps with distinct morphological characteristics such as SwBKP and HwBKP. Both pulp types demonstrated faster strength development through refining than beating, but the extent of improvement varied. SwBKP, with its longer and more flexible fibers, exhibited a more pronounced improvement in tensile and burst strength, consistent with the literature indicating that fiber morphology significantly influences the response to mechanical treatment [27]. HwBKP also benefited from refining, though to a lesser extent, due to its shorter and stiffer fibers. These findings suggest that while the Valley beater remains a useful tool for pre- liminary experiments, its limitations in accurately predicting industrial refining effects, especially for specific pulp types, should be recognized. The structural and mechani- cal precision of refiners is crucial for optimizing pulp strength properties, emphasizing the need for direct refining trials when predicting refining outcomes based on beater ex- periments [24,27]. Overall, this study underscores the advantages of disk refiners over beaters in achieving superior tensile and burst strength outcomes and highlights important implications for refining process optimization in pulp quality improvement. Figure 11 presents the evolution of tear strength in SwBKP and HwBKP as a function of both treatment time (Figure 11a) and freeness (Figure 11b) for beating and refining. Several key trends can be observed: For SwBKP (softwood bleached kraft pulp), tear strength initially increases with both beating and refining, reaches a maximum, and then gradually decreases as the treatment proceeds. The initial rise is attributed to improvements in fiber bonding and network entanglement [24]. As refining or beating continues, excessive fiber shortening and increased bonding density eventually lead to a decline in tear strength, since shorter fibers and denser networks limit the fiber pull-out mechanism that underpins tear resistance.
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( b )
Figure11. ( a ) Comparison of tear strength of paper through beating and refining time; ( b ) comparison of tear strength of paper through freeness. A noteworthy finding is that, at a comparable low freeness (approximately 200 mL CSF), both SwBKP and HwBKP samples subjected to refining demonstrate higher tear strength than those subjected to beating. This advantage is particularly clear for SwBKP. The superiority of refining at this freeness is due to two primary factors. First, Refining reaches the target freeness rapidly with minimal fiber shortening, as confirmed by higher length-weighted fiber length in Figure 8b. Second, the fibers in refined pulp maintain their ability to bridge tear cracks due to their preserved length, while the generated external
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