Sustainability 2023 , 15 , 2850
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obtained at a lubricity of approximately 30 ◦ SR. Therefore, the papermaking ability of the selected pulps was determined after refining for the Schopper–Riegler freeness of 30 ± 1 ◦ SR, which was expected to ensure the maximum strength of pulps and contribute to easy dehydration. In further analyses of papermaking ability, the pulp samples that did not promise products with excellent functional properties due to their characteristics at the very beginning were rejected. Therefore, from amongst all considered wastepaper types, the pulps that were characterized by a freeness of >30 ◦ SR in the unrefined state were rejected. In addition, pulp samples in which the content of extractive substances exceeded 1.30% were excluded. Owing to their high chemical reactivity as well as the significant viscosity and adhesion of their components, extractives may hinder the papermaking process in the form of so-called resin difficulties and may create resin stains, visible as spots in the finished products. In addition, pulp samples with fiber length of <900 μ m were excluded as they increase static strength but decrease tear resistance. In contrast, the intermediate fraction lowers the static strength but improves he dynamic strength. Meanwhile, the long-fiber fraction improves all strength properties of the paper. A longer fiber length increases the tear resistance and extensibility of the paper, which are particularly desirable in the case of sanitary papers. In the present study, white wastepaper 1.3, 1.4, 2.1 and 3.2 and mixed wastepaper 1.5 met the above criteria. Considering the practical technological aspects of subsequent processes on a paper machine, the most important changes occurring as a result of the refining process, in addition to from freeness, include the development of swelling degree (WRV) of the recycled pulp. The greater the degree of pulp swelling, the greater the compactness of the structure of the obtained paper, which significantly improves its properties but reduces its dynamic strength. After refining, WRV of the tested pulps increased by 28–61% (Table 2), achieving the highest values for white wastepaper. Mixed wastepaper 1.5 was much more keratinized and achieved a much lower degree of pulp swelling. Similarly, regarding fiber length, higher values were recorded for white wastepaper (Table 3), suggesting that the paper obtained from these types will achieve higher strength parameters at the outset. To date, the highest content of the fine fraction amongst refined pulps has been recorded for white 2.1 and mixed 1.5 wastepaper (Table 2), which is conducive to the quality paper derived from these pulps as well as to the process economy resulting from the saving of bulk additives. Overall, our analyses of unrefined and refined pulps showed that the type of wastepaper (impurity content), similar to chemical composition, does significantly affect fiber and pulp properties.
Table2. Characteristics of wastepaper pulps.
Wastepaper
Fine Content [%inArea]
Fine Content [% in Length]
WRV
Schopper–Riegler Freeness
[%] [ ◦ SR] Unrefined Refined Unrefined Refined Unrefined Refined Unrefined Refined
21.05 23.63 10.03 12.20 14.55 12.39 13.32 16.63 13.42 26.78 17.63 10.71
- -
62.24 65.94 34.73 37.39 49.20 54.99 47.31 51.08 44.98 66.60 46.68 34.53
- -
130.9 136.3 103.8 101.4 103.6 102.3 101.3
- -
36 56 17 18 22 22 20 22 22 35 20 16
- -
White1.1 White1.2 White1.3 White1.4 Mixed1.5 White2.1 White2.2 Mixed2.3 Mixed2.4 Mixed2.5 Mixed3.1 White3.2
15.27
27.89 20.71 41.03 41.93
164.62 159.08 134.17 151.86
31 29 29 30
9.41
15.87 10.95
- - - - -
- - - - -
- - - - -
- - - - -
91.1 97.6
107.2 103.7 105.8
18.47
32.27
185.65
30
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