Appl. Sci. 2025 , 15 , 875
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flocculation could lead to denser fiber packing, explaining the slight reduction in roughness observed for pulp 1.3 at higher dosages. These variations in roughness can influence the final application of the paper. For example, rougher papers, with higher porosity, are advantageous for tissue applications, where air and moisture permeability are desirable. In printing applications, however, such an increase in roughness may pose challenges due to greater ink absorption, potentially compromising print quality. Thus, paper manufacturers must carefully balance retention agent addition to achieve optimal roughness for specific end uses. The differences in roughness trends between wastepaper 1.3 and 3.2 also suggest a potential influence of pulp composition and processing history. Pulp 3.2 consistently exhibits higher roughness values across all retention agent levels, possibly due to differences in fiber morphology or the presence of specific contaminants. These findings underscore the importance of tailoring retention aid dosages to the unique characteristics of the recycled paper furnish. In summary, roughness increases with the addition of retention agents, with distinct differences observed between the two pulp types studied. The results provide valuable insights for optimizing retention agent use in recycled paper production to achieve desired properties while addressing the specific requirements of various paper grades. The tensile properties of wastepaper sheets are critical for determining their suitability in manufacturing and processing [40,41]. Retention aids can enhance fiber retention and inter-fiber bonding, leading to improved tensile strength. However, the effects of these additives are not uniform and depend significantly on their type and concentration, the characteristics of fibers, and other papermaking components. Papermakers carefully select and optimize retention aids to achieve the desired strength properties for different paper types and applications. The tensile properties of the recycled paper sheets are primarily affected by the reten- tion agent’s ability to promote fiber bonding through charge neutralization and flocculation. By reducing the negative charge of the fibers, the retention agent enhances inter-fiber attraction, leading to stronger bonds between fibers. However, excessive dosages of the agent can result in over-flocculation, creating large aggregates that disrupt the uniformity of the fiber network. This imbalance can weaken the tensile properties by introducing structural heterogeneity and reducing the effective load-bearing areas in the paper. The impact of a retention agent based on acrylamide and a cationic derivative of acrylic acid on the tensile properties of two types of wastepaper (1.3 and 3.2) was analyzed. The results, summarized in Tables 2 and 3, reveal complex trends and highlight the non-linear relationship between retention aid concentration and paper strength.
Table2. Tensile properties of paper from 1.3 white wastepaper.
E w
W
W
E b
b
b
E*
Retention Agent Addition I B
F B
W
W T
ε
σ
σ T
T
T
T
[J/m 2 ]
[J/g]
[N/m] [Nm/g] [MPa]
[%] Ref.
[m]
[N]
[N/m] [Nm/g]
[%] 1.60 1.45 0.92 0.88 0.78 0.83 1.08 1.25 1.40
2700 2300 1800 1800 1700 1750 1850 2000 2150
30.9 27.5 20.7 21.3 19.4 19.3 21.3 22.9 24.5
2095 1853 1425 1431 1338 1353 1465 1578 1685
26.4 22.8 17.9 17.7 16.5 17.0 18.1 19.5 21.2
22.1 17.9
0.278 320,667 4033 0.220 305,500 3753 0.105 295,167 3693 0.093 292,383 3611 0.089 289,817 3579 0.085 288,333 3622 0.136 283,483 3501 0.154 279,767 3455 0.194 276,833 3477
2915 2777 2683 2623 2600 2622 2543 2510 2515
0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0
8.4 7.6 7.2 6.7
11.0 12.5 15.5
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