Sustainability 2023 , 15 , 2850
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This was also true for optical parameters. Interestingly, compared to mixed wastepaper, white wastepaper did not exhibit higher whiteness indices. The optical characteristics are presented in Table 4 and Figures 2 and 3.
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Figure 2. Light reflection curve of white wastepaper.
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Figure 3. Light reflection curve of mixed wastepaper.
Another important property that contributes significantly to the functional properties of paper is air permeability. Moreover, air permeability is an important indicator for the production process control of sanitary papers, since it reflects the porosity of the product and thus its absorbance and tensile properties. Most of the studied papers produced from unrefined pulps were characterized by high air permeability, which decreased significantly after refining (Table 4). However, whether this is disadvantageous depends on the purpose of application. The tensile properties of paper sheets are important for manufacturing and process- ing [51,52]. Therefore, the effects of specific pulp parameters and impurity contents on the strength of papers were analyzed, and the results are summarized in Tables 5 and 6. Regarding strength properties, papers produced from unrefined white wastepaper 2.1 and 3.2 achieved the best results in the conducted tests (Table 5). Refining significantly improves the papermaking ability of wastepaper pulp, increasing the strength of the obtained papers. From refined pulp, the best results were recorded for white recycled papers 1.3 and 3.2 (Table 6), which was expected considering their excellent pulp and fiber properties (Tables 2
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