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Freeness WRV (°SR) (%)
Sample dimensions (mm)
Sample area (mm 2 )
1 × 1 2 × 2
1 4 5 9
51 42 39 31 31 30 29 28 28 26 25 25 25 25 25
214.5 (3.4) 212.5 (2.2) 211.1 (1.2) 208.5 (1.6) 208.1 (1.3) 207.2 (1.4) 206.8 (1.2) 206.3 (1.5) 206.1 (1.9) 205.2 (2.5) 205.6 (1.8) 204.8 (2.2) 203.6 (2.8) 202.8 (1.6) 201.2 (3.1)
1 × 5 (P-7)
3 × 3
1 × 10 (P-6)
10 16 25 30 36 64
4 × 4 5 × 5
2 × 15 (P-5)
6 × 6 8 × 8
10 × 10
100 150 320 400
6 × 25 (P-4)
P-3
20 × 20
Reference
–
Table 2. Characteristics of refined cellulose pulps. Standard deviation are given in brackets.
The average degree of polymerisation (DP) of cellulose in each pulp was determined using the viscometric method described in ISO 5351 (2010). In industrial conditions, the freeness index of pulp is a commonly used parameter to assess the degree of refin- ing of pulp based on how easily it dewaters. The dewatering of pulp on a paper machine screen is a very useful indicator in industrial practice; therefore, in this study, the effect of the initial fibre length on changes in this indicator was examined. Table 2 shows that the freeness of the refined pulp decreases as the initial fibre length increases, which is attributed to the decreasing amount of fine material in the pulp, in line with the current state of knowledge 23–28 . It is assumed that a freeness index of about 30°SR is optimal for most papermaking properties 29,30 . However, obtained results do not indicate that a freeness of ~ 30°SR necessarily achieves a maxi- mum tensile strength (Table 2). This confirms that there is no straightforward relationship between freeness and paper properties. Therefore, the freeness of the pulp is not useful to compare the papermaking potential of pulp with different initial parameters. The impact of initial fibre length on internal fibrillation, one of the basic effects of refining 31–35 , was also stud- ied. Progress in achieving internal fibrillation of refined fibres is commonly assessed based on an increase in their swelling 36,37 , usually measured by the WRV 38–40 . The analysis in Table 2 indicates that the WRV increases as the initial fibre length decreases, reaching a maximum value of 214.5%. This increase in fibre swelling is accompanied by an increase in the density of the paper, which in turn increases the resistance of the paper to air permeability (Table 2), consistent with previous studies 8,41–43 . The initial fibre length did not affect the DP of the produced pulps. The DP was 931 ± 0.89 regardless of the initial dimensions of the samples tested. This confirms previous findings in the literature that mechanical treat- ment has little effect on the DP of cellulose 44,45 . Table 3 and Fig. 1 show the morphological characteristics of the fibres of the examined pulps. The fibre length for the refined pulps is characterised by lower values compared to the unrefined pulps, which confirms that one of the basic effects of refining is fibre shortening 46–48 . It should be noted that the use of mean weighted or mean geometric fibre length eliminates the influence of the fines fraction on the analysis result 49,50 . The results presented in the Table 3 show that the examined parameter values of fibres and pulps (mean fibre width, mean fibre coarseness, macro fibrillation index, broken fibre content, and fine content) tend to decrease with decreasing length of the initial fibres, and consequently, the mean weighted fibre length increases. However, an initial sample length greater than 5 mm does not significantly affect the examined fibre properties and fine content. It is therefore likely that a strip width of 5 mm is the limit above which significant changes in the pulp do not occur. The results are similar before and after the refining process (Fig. 1). Microscopic images of the refined pulps, recorded using a Morfi Compact Black Edition camera, are shown in Fig. 2. The decrease in fibre length after the model pulp recycling process is proportional to the decrease in the dimensions of the pre-cut pulp samples. The most significant fibre shortening is noticed for the 1 × 1 mm sample. There is no significant difference in fibre dimensions for the 10 × 10 mm samples and the reference sample (Fig. 2). Therefore, it is possible to shred paper in a shredder to a specific level of fragmentation without fear of excessive shortening of the fibres, which would make the production of high-quality paper more difficult. According to previous findings, fibre length has a significant impact on the paper stretch index 51 , and excessive fibre shortening and a high fines content cause the paper product to become rigid and reduce its deformability 49,52,53 . Our results, however, indicate that the stretch of the examined papers is similar, irrespec- tive of the fibre length and fines fraction content (Table 4), in contrast to previously reported results. However,
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