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Table 3. Fibre and pulp properties after model pulp recycling process including refining. Standard deviation are given in brackets. Sample area Mean fibre width Mean fibre coarseness Macro fibrillation index Broken fibre content Fine content mm 2 μm mg/m % % % in area 1 32.9 (0.1) 0.207 (0.017) 1.135 (0.047) 52.3 (1.1) 11.70 (0.54) 4 32.4 (0.3) 0.199 (0.009) 1.085 (0.007) 50.0 (0.5) 9.42 (0.13) 5 32.5 (0.2) 0.197 (0.008) 1.059 (0.009) 49.3 (0.6) 9.20 (0.19) 9 31.9 (0.1) 0.190 (0.011) 0.890 (0.023) 45.8 (0.4) 6.85 (0.24) 10 31.8 (0.2) 0.195 (0.007) 0.903 (0.020) 46.1 (0.3) 6.78 (0.18) 16 31.5 (0.1) 0.196 (0.006) 0.861 (0.011) 44.9 (0.8) 5.98 (0.18) 25 31.7 (0.4) 0.195 (0.001) 0.832 (0.007) 43.8 (1.5) 5.51 (0.14) 30 31.5 (0.3) 0.193 (0.002) 0.819 (0.006) 43.6 (1.1) 5.26 (0.16) 36 31.2 (0.2) 0.191 (0.008) 0.814 (0.005) 43.0 (0.6) 5.08 (0.28) 64 31.5 (0.1) 0.187 (0.009) 0.799 (0.012) 41.5 (0.2) 4.26 (0.14) 100 31.1 (0.1) 0.182 (0.012) 0.764 (0.022) 39.3 (0.6) 3.25 (0.21) 150 31.0 (0.3) 0.184 (0.011) 0.769 (0.015) 39.8 (0.4) 3.04 (0.18) 320 30.8 (0.1) 0.183 (0.010) 0.752 (0.012) 38.4 (0.4) 2.95 (0.10) 400 30.9 (0.4) 0.184 (0.001) 0.758 (0.058) 39.2 (0.5) 3.00 (0.60) Reference 30.7 (0.3) 0.180 (0.010) 0.777 (0.024) 35.4 (0.6) 2.81 (0.60)
Figure 1. Average fibre length dependency on the cut surface area of samples (before and after model pulp recycling process including refining).
the results listed in Table 4 show that fibre length exerts a significant impact on the dynamic tensile properties of paper 54–56 . Importantly, in the case of fibre properties (Table 3 and Fig. 1), changes in the area of the shred- ded samples above 25 mm 2 did not significantly affect the tensile paper properties (Table 4). Based on this, it can be concluded that samples can be cut at 5 mm or larger without significant shortening of the fibres and no significant changes in pulp and paper properties. Therefore, the paper shredding provides useful wastepaper when performed in devices up to class P-6. Research has shown that the fines fraction produced from fibres is responsible for slowing pulp-dewatering in the forming section of a paper machine 57,58 . The results obtained are fully consistent with those of earlier studies, in that the pulps with the highest fines content also have the highest SR freeness values (Table 2). The air permeability of paper decreases as the SR freeness level increases 59–62 , in agreement with previous results. The pulps characterised by lower average fibre length, and therefore, have improved barrier properties to gases even though their tensile properties are reduced (Table 4). The roughness of paper increased with increasing initial fibre length, in agreement with previous results 34,61,63,64 . Therefore, the best smoothness results (230 mL/min) were obtained for paper produced from the pulp with the lowest fibre length. From the data provided, it can be concluded that this paper would likely have the best printability. Microscopic images of the paper sheets, recorded using a Keyence VHX-6000 microscope equipped with a VH-Z100UR lens, are presented in Fig. 3. The fibres in the reference pulp (Fig. 3a) appear undamaged. The paper obtained from samples cut into 5 × 5 mm pieces (Fig. 3b) shows both undamaged fibres and cut fibres.
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