# Impact of Retention Agents on Functional Properties of Recycled Paper in Sustainable Manufacturing
## 3 Results and Discussion  
Table 1 presents the structural properties of papers made from recycled pulps with varying additions of a retention agent. Among these properties, air permeability stands out as a critical factor influencing both the functional performance of the paper and its production process control, especially in applications such as sanitary papers. Air permeability quantifies the ease with which air can pass through the paper material. Higher values indicate a \"tighter\", less permeable sheet that resists airflow, while lower values are associated with greater porosity, allowing air to traverse the structure more readily. The porous structure of paper is a key determinant of its utility in numerous applications, as it affects liquid absorption, moisture release, and the exchange of gases or vapors. In the current study, the results did not indicate that the amount of retention agent added had a significant impact on the air permeability of the tested papers, as in all cases, the air permeability exceeded the measurement range of the Bendtsen device (Table 1). This confirms the highly porous nature of the samples and suggests that, within the studied range of retention agent dosages, the effect on porosity and structural integrity of the paper was not significant enough to be detected. The exceptionally high air permeability observed in all samples can be advantageous or disadvantageous depending on the intended application of the paper. For example, in sanitary papers or absorbent packaging materials, higher air permeability is a desirable feature. It enhances the paper's ability to absorb fluids rapidly and contributes positively to properties such as softness, fluid handling efficiency, and overall user comfort. In printing processes, such as offset or gravure printing, air permeability affects ink transfer and drying rates. Papers with specific air permeability properties may be preferred to achieve optimal print quality and minimize issues like ink set-off or poor ink adhesion. For certain paper grades like packaging materials, reduced air permeability can be desirable, as it enhances the material's resistance to gas, vapor, and moisture transmission, thus improving its barrier properties. While higher air permeability can offer benefits in terms of porosity and absorbency, it may also lead to reduced mechanical strength in some cases. Papers with high porosity, while excellent in terms of absorption and fluid handling, may exhibit reduced tensile strength and tear resistance. This trade-off underscores the importance of optimizing air permeability in conjunction with mechanical properties to meet specific application requirements. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{3}{c}{**Air Permeability**} \\ \cline{2-3}
**Retention Agent Addition [\%]** & \multicolumn{2}{c}{**[mL/min]**} \\ \cline{2-3}  & **1.3** & **3.2** \\ \hline Ref. & 5000 & 5000 \\
0.1 & 5000 & 5000 \\
0.2 & 5000 & 5000 \\
0.3 & 5000 & 5000 \\
0.4 & 5000 & 5000 \\
0.5 & 5000 & 5000 \\
0.6 & 5000 & 5000 \\
0.8 & 5000 & 5000 \\
1.0 & 5000 & 5000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structural properties of papers. While retention agents are generally known to influence fine fiber and filler retention, their negligible impact on air permeability in this specific study underscores the importance of other factors. These include the composition of the recycled pulp or the manufacturing process parameters, which may have a more pronounced effect on porosity and related properties. Understanding and optimizing these factors is critical when tailoring air permeability to align with target properties like absorbency, printability, or barrier performance. In summary, the structural properties of the studied papers highlight the interplay between air permeability, porosity, and other functional attributes. By carefully adjusting the type and dosage of retention agents, it is possible to tailor paper properties to meet specific performance criteria for diverse applications.