Appl. Sci. 2025 , 15 , 9160
11of 14
fibrillation supports sufficient fiber bonding without excessive densification that would otherwise impede tearing [18,24,27]. Conversely, beating requires a prolonged treatment to achieve the same freeness, resulting in more fiber cutting and network damage. The consequence is shorter fibers and a reduced capacity for reinforcing the paper matrix, producing lower tear strength than refining at the same drainage level. It is also important to note that, while longer fibers generally confer higher tear strength, this relationship depends on achieving an optimal balance between fiber length, fiber bonding, and network structure. Over-refining or excessive beating, even for softer pulps, can lead to increased bonding but decreased fiber pull-out, resulting in a reduction in tear strength beyond the optimum. For HwBKP, tear strength is consistently lower than for SwBKP due to the inherently shorter and stiffer nature of hardwood fibers [32]. Still, the refined hardwood samples retain slightly higher tear strength than beaten samples at equivalent low freeness, for the same reasons described above. In summary, Figure 11 shows that refining with an ultra-fine bar plate not only preserves fiber length but also achieves higher tear strength at low freeness compared to beating. This outcome supports the theory that under optimal conditions, refining is more advantageous than beating for maximizing tear resistance, especially in softwood pulps [24,29]. 3.4. Observation of Fiber Morphology Before and After Mechanical Treatment Figure 12 displays optical microscopy images that illustrate the microstructural evo- lution of both HwBKP and SwBKP fibers subjected to disintegration, Valley beating, and ultra-fine bar refining. All samples were evaluated at similar freeness levels (approximately 200 mL CSF), enabling a direct comparison of morphological effects due to the different mechanical treatments. In the disintegrated states ((a) and (d)), both HwBKP and SwBKP fibers appear long and relatively unaltered, with little to no external fibrillation and a scarcity of fines. These images represent the baseline fiber condition prior to mechanical processing. Following the Valley beating ((b) for HwBKP, (e) for SwBKP), fibers from both pulps exhibit noticeable external fibrillation as well as increased fragmentation, with shorter fibers and a higher abundance of fines and detached fiber fragments. This reflects the tendency of beating to induce not only fibrillation but also significant fiber cutting, particularly visible in the formation of curled fragments and debris. In contrast, the refined pulp images ((c) for HwBKP, (f) for SwBKP) demonstrate extensive fibrillation along relatively intact and elongated fibers. The ultra-fine bar refiner treatment promotes the development of a dense layer of fibrils on the fiber surfaces, enhancing their ability to form strong inter-fiber bonds while largely preserving fiber length and morphology. Less fiber breakage and fewer fines are observed than in the beaten samples, especially for SwBKP, where the network remains open and fiber integrity is evident. Taken together, Figure 12 visually confirms that ultra-fine bar refining achieves sub- stantial fibrillation and surface area enlargement without the excessive fiber shortening and fragmentation typical of Valley beating. This structural preservation is especially bene- ficial for mechanical properties (tensile, burst, and tear strengths) in the resulting paper, supporting the quantitative findings of the study. Moreover, the images highlight that the impact of mechanical treatment on fiber morphology varies with pulp type, with softwood fibers generally retaining more length and flexibility than hardwood fibers, regardless of the process.
Made with FlippingBook interactive PDF creator