Impact of Retention Agents on Functional Properties of Recycled Paper in Sustainable Manufacturing

Abstract

The proper dosing and optimization of retention additives are necessary to ensure the desired benefits without compromising other aspects of the paper manufacturing process. In this study, the effects of a cationic polyelectrolyte based on acrylamide and a cationic derivative of acrylic acid on the different properties of paper containing recycled fibers were investigated. The structural and tensile properties were examined through various analyses to determine the optimal dosage of the retention additive. The results obtained indicate that while the retention agent can enhance papermaking efficiency by improving retention, drainage, and sheet formation, it also negatively impacts the tensile strength and surface smoothness of the recycled paper. This complexity highlights the importance of a balanced approach in optimizing retention aid dosages. Determining the optimum dosage of such an agent requires multiple trials and analyses with varying dosages. This review aims to offer a background for engineers seeking to enhance the competitiveness and reduce production costs of their paper products, as well as for researchers striving to surpass the existing standards and achieve innovative outcomes.

1. Introduction

In recent years, the paper industry has increasingly embraced sustainable practices, driven by both economic and environmental considerations. The inherent sustainable nature of papermaking—recycling fibrous materials and reducing resource consumption—positions it as a key industry for achieving circular economy objectives. However, to fully realize its potential as a sustainable industry, there is a pressing need to adopt greener, more efficient processes that minimize the environmental impact and resource use. As highlighted in recent studies [1], advancements in process technology, along with innovative chemical additives, play a critical role in achieving these goals.

The role of retention aids in this context is particularly significant. Paper production, especially from recycled pulp, requires support from various auxiliary chemicals. Functional chemicals not only directly affect the properties of the paper but are also added to increase production efficiency and improve the operation of the paper machine. Retention aids are among the chemicals used to support the process.

In paper production, the goal is to achieve the highest possible retention of fine fractions. High retention efficiency during the paper-formation process is crucial for the effectiveness of papermaking systems, as it significantly impacts both the operation of the paper machine and the quality of the finished product. However, during the formation of the paper web, only a portion of the fine fraction is retained in the paper. At the same time, the rest is lost in white water through the mesh due to high turbulence and high shear forces, resulting in a relatively low retention rate. Retention aids introduced into the pulp are deposited on the surface of fibers and fine fraction, neutralizing their negative charge. As a result of weakening the repulsive forces of the individual components of the pulp, the fine fraction agglomerates, adsorbs onto the fiber surface, and causes flocculation of the pulp [2]. These changes lead to several benefits, such as the increased retention of the fine fraction, reduced losses of pulp components, improved pulp dewatering (resulting in higher efficiency and lower energy requirements), and improved water management in the paper mill. Therefore, high retention is important from both an economic perspective (reducing additive losses) and an environmental standpoint (reducing wastewater emissions and potential environmental risks from chemicals used). Low retention, in turn, can result in numerous issues, such as reduced runnability, increased buildup of deposits, defects in the paper sheet, higher costs for additives, and more frequent downtime [3]. The proper application of retention aids is also an efficient method to minimize environmental pollution and conserve resources by retaining fines and fillers.

In recent years, extensive research on new types of retention aids has produced a vast number of fundamental [4,5,6] and applied studies [7,8,9] owing to their substantial impact on the quality of paper stock and the operational efficiency of paper machines.

Based on the composition of retention aids, retention systems are typically classified as single-component or dual-component systems. Traditional single-component systems include both inorganic and organic retention aids. Common inorganic retention aids include aluminum sulfate (Al₂(SO₄)₃) [10], polyaluminum chloride (PAC) [11], and ferric chloride (FeCl₃) [12,13]. Organic retention aids are further categorized into natural polymers and synthetic polymers, which differ in the structure of their basic units, degree of polymerization, and the type and density of electric charge, which also significantly influences their mechanism and effectiveness.

Natural polymers are extensively utilized due to their affordability, renewability, and environmental benefits [14]. Commonly used natural polymers in papermaking, derived from various sources, include notable materials such as starch, chitosan, guar gum, and cellulose. Among these, starch is the most prevalent because of its abundant availability and low cost [15], but also chitosan [16,17,18,19,20,21]. However, the low charge density and inconsistent structure of natural polymers limit their applications in the papermaking process. Consequently, significant research has been directed toward synthetic polymers to address these limitations. Currently, the most commonly used synthetic polymers in papermaking are polyacrylamide (PAM) and polyethylene imine (PEI) [22,23,24]. Synthetic polymers are effective but not easily degradable in conventional water treatments, leading to their accumulation in water systems.

Good retention of the fine fraction does not go hand in hand with the desired dewatering of the pulp [25]. Rapid dewatering reduces the retention of both the fine fraction and fillers and costly bulk additives. On the other hand, improving retention can negatively impact the dewatering of the pulp and paper transparency. To improve retention and dewatering, dual-component systems with a high electrostatic charge and low molecular weight of the cationic material (dual polymeric retention system) have been developed to coagulate fibers and solid particles, followed by the use of a high-molecular-weight polymer with weak cationic charge (usually polyacrylamide) [26] (microparticle retention-aid system). The danger of using the first system is either excessive cationization of the suspension, leading to the electrostatic repulsion of fibers and solid particles and deterioration of retention, or excessive flocculation. Large flocs improve dewatering but significantly compromise the uniformity of transparency and often affect the strength properties by creating weak areas in the web. So, the microparticle retention aid system has become one of the most successful products available on the market. Compared to conventional retention systems, it not only provides outstanding retention of wet-end chemical additives, fine fibers, and fillers but also decreases the concentration of white water and boosts the speed of the paper machine, thereby offering both environmental and economic advantages [27,28].

The proper dosing and optimization of retention agents are necessary to ensure the desired benefits without compromising other aspects of the paper manufacturing process. By improving process efficiency and reducing raw material losses, retention aids contribute to both the economic viability and environmental sustainability of paper manufacturing. However, optimizing their use requires careful balancing to avoid adverse effects on the physical and mechanical properties of the paper. While the influence of cationic polyelectrolytes on papermaking processes has been widely studied [29,30], this study focuses on a novel aspect—the optimization of retention aid dosage specifically for recycled pulp under industrial conditions. This study evaluated the impact of a retention aid on the properties of recycled paper and selected the optimal addition of the aid to the recycled fiber stock. This practical application distinguishes our work from previous studies by directly addressing the challenges faced by the paper industry, including balancing the retention efficiency with the mechanical and surface properties of paper. By providing insights into real-world implementation, this research contributes to both academic understanding and practical advancements in sustainable papermaking.

2. Materials and Methods

2.1. Fibrous Materials and Retention Agent

For this research, white wastepaper sourced from a paper mill was selected. This included products made from bleached pulps, specifically scraps of wood-free waterproof paper with minimal print and no adhesive or colored components, classified as grade 3.04 in accordance with the EN643 “List of European standard types of wastepaper” [31]. For comparison, two additional grades of white wastepaper were chosen, labeled as 1.3 and 3.2. Previous analyses on these types of wastepaper [32] demonstrated that it was feasible to eliminate at least 80% of the fine mineral fraction, achieving an average pulp recovery rate of 76.5% along with the best strength properties. These grades also exhibit very similar chemical compositions and impurity levels, as established in earlier studies [32].

The wastepaper delivered from the paper mill was segregated into uniform fractions, manually shredded, and thoroughly mixed to ensure the homogeneity of the sample. The prepared wastepaper was then sealed in polypropylene foil bags and stored in tightly sealed barrels to prevent moisture and contamination. After mechanical shredding, the samples were packed into airtight containers and kept at a stable temperature of around 15 °C.

As a retention aid, the study utilized a cationic polyelectrolyte derived from acrylamide and a cationic acrylic acid derivative. This additive was incorporated into the secondary pulp in varying amounts, ranging from 0.1% to 1.0%.

2.2. Preparation of Pulps

The preparation process of the pulps for further research involved cleaning, screening, and washing, optimized based on prior studies on wastepaper. These steps enabled the removal of fine mineral fractions and impurities that could negatively impact the washing process of the wastepaper pulp or accelerate wear or damage to the screen. It is worth noting that the pulps used for the tests were free from heavy impurities (such as sand, paper clips, etc.), which were separated during the initial sorting of wastepaper samples.

The first step involved defibering the pulp in a laboratory vortex pulper. Rewetted pulp samples (22.5 g dry weight, soaked in water for 24 h) were disintegrated using a JAC SHPD28D laboratory propeller pulp disintegrator (Danex, Katowice, Poland) at 23,000 revolutions, following ISO 5263-1 (2004) [33]. After defibering, the pulps underwent a purification process in a laboratory hydrocyclone with a diameter of 60 mm, operated at a pressure difference of 1.5 bar. Next, the cleaned pulp was subjected to a screening process using a PS-114 membrane screener (Danex, Katowice, Poland) with an amplitude of 25 mm and a frequency of 2 Hz. The screener was equipped with a 0.15 mm gap screen. During this process, the fibrous fraction (designated after the process as sorted pulp) flowed through the sorting plate under the hydrostatic pressure of the water column and the movement of the vibrating membrane beneath the screen plate. After screening, the fibrous fraction was discharged through an overflow spigot into a container below, where it was drained on a sieve. The rejects remained on the sorting plate and were removed after each screening cycle.

2.3. Washing of Wastepaper

The washing process involved multi-stage dewatering, during which impurities smaller than the screen openings were removed, including fine fractions, fillers, shorter fibers, and other small particles. This process was performed using a screener equipped with a 180-mesh sieve (90 µm openings). The pulps were washed with a controlled volume of water while being gently mixed to prevent the formation of a filter layer on the screen plate.

2.4. Preparation of Paper Sheets

Laboratory paper sheets were produced from the screened and washed wastepaper pulps. The sheets were formed using a Rapid-Koethen apparatus (Danex, Katowice, Poland) in compliance with PN-EN ISO 5269-2 (2007) [34]. Each sheet had a base weight of 80 g/m², as per ISO 536:2019 [35]. Only sheets with a basis weight within the range of 79–81 g/m² were selected for further testing.

Prior to testing, the paper samples were conditioned for at least 24 h at a temperature of 23 °C and 50% relative humidity, in accordance with ISO 187:2022 [36].

2.5. Analysis of the Paper Properties

The surface roughness of the paper was measured in accordance with ISO 8791-2:2013, using the TMI 58-27 Bendtsen Roughness Tester (Kontech, Lodz, Poland) [37]. Air permeability was evaluated using the same device, following the guidelines of ISO 5636-3:2013 [38].

The key strength properties of the paper were assessed using a Zwick 005 Pro-Line testing machine (ZwickRoell, Ulm, Germany), in compliance with PN-EN ISO 1924-2:2010 [39], with data acquisition and analysis performed via testXpert III software. The tensile properties of the paper were tested as follows:

• I_B: breaking length [m].
• F_B: tensile force at break [N].
• σ_T^b: width-related force at break [N·m⁻¹].
• σ_T^W: force-at-break index [Nm·g⁻¹].
• ε_T: strain at break [%].
• W_T^b: energy absorption [J·m⁻²].
• W_T^W: energy absorption index [J·g⁻¹].
• E^b: tensile stiffness [N·m⁻¹].
• E^w: tensile stiffness index [Nm·g⁻¹].
• E*: Young’s modulus [MPa].

A detailed statistical analysis was conducted for each research series, calculating key indicators such as the arithmetic mean, standard deviation, and percentage relative error.

3. Results and Discussion

Table 1. Structural properties of papers.

Retention Agent Addition [%]	Air Permeability
	[mL/min]
	1.3	3.2
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

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.

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. This approach underscores the importance of a holistic understanding of material interactions in recycled paper production.

Another tested parameter, i.e., roughness, is closely related to porosity. The tests performed indicate that the roughness of reference papers (406 μm for 1.3 wastepaper and 553 μm for 3.2 wastepaper) is significantly lower compared to the samples with the highest addition of retention agent, where the maximum roughness reached 1281 μm and 1084 μm for pulps 1.3 and 3.2, respectively. This significant increase highlights the impact of retention agent dosage on paper structure.

The data (Figure 1) show a general trend: increasing the addition of the retention agent typically leads to higher roughness, although certain deviations are observed. For 1.3 wastepaper, roughness increases steadily from 525 μm at a 0.1% addition to a peak of 1281 μm at 0.5% before declining slightly at higher dosages. Conversely, for 3.2 wastepaper, roughness increases more linearly, reaching its maximum at 1.0% retention agent addition (1084 μm).

This behavior may be attributed to the interaction between the retention agent and fiber structure. According to research, rougher paper surfaces can result from uneven distribution of fines and fillers caused by excessive retention agent usage. Initially, the retention agent facilitates the formation of a more porous network by promoting better flocculation, which increases roughness. However, beyond a certain dosage, excessive 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 retention 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

Table 2. Tensile properties of paper from 1.3 white wastepaper.

Retention Agent Addition	IB	FB	σTb	σTW	εT	WTb	WTW	Eb	Ew	E*
[%]	[m]	[N]	[N/m]	[Nm/g]	[%]	[J/m2]	[J/g]	[N/m]	[Nm/g]	[MPa]
Ref.	2700	30.9	2095	26.4	1.60	22.1	0.278	320,667	4033	2915
0.1	2300	27.5	1853	22.8	1.45	17.9	0.220	305,500	3753	2777
0.2	1800	20.7	1425	17.9	0.92	8.4	0.105	295,167	3693	2683
0.3	1800	21.3	1431	17.7	0.88	7.6	0.093	292,383	3611	2623
0.4	1700	19.4	1338	16.5	0.78	7.2	0.089	289,817	3579	2600
0.5	1750	19.3	1353	17.0	0.83	6.7	0.085	288,333	3622	2622
0.6	1850	21.3	1465	18.1	1.08	11.0	0.136	283,483	3501	2543
0.8	2000	22.9	1578	19.5	1.25	12.5	0.154	279,767	3455	2510
1.0	2150	24.5	1685	21.2	1.40	15.5	0.194	276,833	3477	2515

and

Table 3. Tensile properties of paper from 3.2 white wastepaper.

Retention Agent Addition	IB	FB	σTb	σTW	εT	WTb	WTW	Eb	Ew	E*
[%]	[m]	[N]	[N/m]	[Nm/g]	[%]	[J/m2]	[J/g]	[N/m]	[Nm/g]	[MPa]
Ref.	3750	42.6	2932	36.2	2.53	54.2	0.669	408,133	5040	3662
0.1	3200	36.6	2524	31.2	2.37	46.0	0.568	383,817	4740	3444
0.2	2600	29.3	2018	24.9	1.42	21.2	0.262	376,967	4655	3382
0.3	2450	27.6	1899	23.4	1.35	17.9	0.221	373,000	4606	3346
0.4	2450	27.7	1907	23.5	1.23	18.3	0.226	372,867	4604	3345
0.5	2450	27.8	1912	23.6	1.33	16.4	0.203	365,733	4516	3281
0.6	2600	29.4	2022	25.0	1.67	27.2	0.336	356,433	4401	3198
0.8	2600	29.7	2048	25.3	1.97	30.0	0.371	354,900	4383	3184
1.0	3050	34.8	2395	29.6	2.13	37.7	0.466	347,217	4288	3115

, reveal complex trends and highlight the non-linear relationship between retention aid concentration and paper strength.

For the 1.3 wastepaper, the reference sample exhibited the highest tensile strength values, with a breaking length of 2700 m. The addition of the retention aid resulted in an initial decrease in these values. At an addition rate of 0.2%, the breaking length dropped to 1800 m (a 33% reduction) and the force-at-break index decreased to 17.9 Nm/g (a 32% reduction). Further increasing the retention agent to 0.3% and 0.4% caused slight variations, but no significant improvement was observed. Interestingly, at concentrations above 0.8%, a reverse trend emerged, with strength values starting to recover. At 1.0% addition, the breaking length increased to 2150 m, and the force-at-break index reached 21.2 Nm/g. Despite this recovery, these values still did not exceed those of the reference sample. Other mechanical properties followed a similar trend. Strain at break decreased significantly at lower dosages, from 1.60% in the reference to 0.92% at 0.2%. However, it improved to 1.40% at 1.0%. The energy absorption index showed a marked drop at lower dosages but recovered to 0.194 J/g at 1.0%.

The 3.2 wastepaper exhibited higher initial tensile properties compared to 1.3, likely due to differences in pulp composition and the heterogeneity of wastepaper. The reference sample had a breaking length of 3750 m, a breaking force of 42.6 N, and a tensile index of 36.2 Nm/g. However, the addition of the retention agent followed a similar decreasing trend. At 0.2% addition, the breaking length reduced to 2600 m (a 31% reduction), and the force-at-break index decreased to 24.9 Nm/g (a 31% reduction). As with the 1.3 type of wastepaper, increasing the retention agent concentration to 0.8% and 1.0% reversed the trend, with the force-at-break index reaching 29.6 Nm/g at 1.0%. Despite this recovery, the values remained lower than those of the reference. The strain at break and work of fracture also exhibited comparable trends. At 0.2%, the strain at break dropped sharply to 1.42% but improved to 2.13% at 1.0%. Similarly, the energy absorption index decreased to 0.262 J/g at 0.2% but recovered to 0.466 J/g at 1.0%.

The results indicate a non-linear relationship between retention agent dosage and tensile properties, with an optimal range where the balance between fiber retention and bonding strength is achieved. Both wastepaper types exhibited a notable decline in tensile properties at lower concentrations of the retention agent (0.2–0.6%), with an average reduction in breaking length of 36% for 1.3 and 31% for 3.2. This reduction can be attributed to the over-dosage of the cationic polyelectrolyte, which may lead to flocculation and uneven fiber distribution, weakening the paper’s structure, which aligns with the findings from other studies, underscoring the importance of optimizing retention agent levels to achieve desired mechanical performance without compromising structural uniformity. Interestingly, at higher concentrations (0.8–1.0%), the tensile properties showed recovery. This suggests that the retention aid’s ability to enhance fiber bonding overcomes its adverse effects at higher dosages, although the recovered values did not surpass the reference.

The higher strength values observed for the 3.2 wastepaper suggest greater fiber quality or heterogeneity compared to 1.3. This indicates that the effects of retention aids are influenced not only by their concentration but also by the intrinsic properties of the wastepaper.

The study highlights the importance of optimizing retention aid dosage to balance its benefits and drawbacks. An excessive or insufficient addition can negatively impact tensile properties. While the 0.8–1.0% range provides a reasonable compromise for strength recovery, further refinements in dosage may be needed to maximize the performance for specific paper grades. The results underline the necessity of tailoring retention agent strategies to the type of wastepaper and desired paper properties, ensuring efficient and sustainable papermaking processes.

It should also be noted that higher strength values were observed for the 3.2 wastepaper, indicating the heterogeneity of the raw material (Table 2 and Table 3).

The conclusions of the conducted study highlight the need for further innovation in the field of retention aids to improve paper properties without negatively impacting production processes. Future development directions include the creation of retention aids tailored to specific fiber types and wastepaper streams, which will help minimize adverse effects on the mechanical and surface properties of paper. Simultaneously, research is focusing on multifunctional additives, such as nanocellulose and biopolymers, which can simultaneously enhance retention, mechanical strength, and surface quality [42]. A critical element of future solutions is the implementation of advanced process control systems and artificial intelligence, enabling the real-time optimization of wet-end chemistry to ensure consistency and high production quality. Another key direction is the development of biodegradable and non-toxic retention aids, meeting the needs of sustainable development and the growing demand for eco-friendly products. Furthermore, the use of hybrid retention systems, nanotechnology, and advanced modeling tools opens new opportunities to improve the efficiency of production processes and the quality of final products. These innovative approaches will allow the paper industry to meet the increasing environmental, quality, and economic demands.

4. Conclusions

Overall, the addition of a retention agent can significantly enhance papermaking efficiency by improving key process parameters such as fiber retention, drainage, sheet formation, and uniformity. Optimized retention systems can lead to achieving higher-quality paper products with reduced raw material losses, minimized environmental impact, and improved operational cost-effectiveness. These benefits are crucial for maintaining competitiveness in an industry where cost and sustainability are critical considerations.

However, the conducted analyses demonstrate that while the cationic polyelectrolyte based on acrylamide and a cationic derivative of acrylic acid contributes positively to process efficiency, it can adversely affect certain mechanical and surface properties of recycled paper. Specifically, this study revealed a marked decline in tensile strength and strain at break, with reductions of up to 36% for breaking length and 51% for strain compared to reference samples. Additionally, increased roughness parameters observed in the study suggest that the retention aid adversely affects surface smoothness, an important property for applications requiring high-quality printability or tactile smoothness, such as sanitary papers.

The observed trade-offs highlight the importance of optimizing retention aid dosages to balance production efficiency and the functional properties of recycled paper products. While lower dosages improve tensile properties and surface quality, they may compromise retention and drainage efficiency. Conversely, higher dosages enhance retention but can degrade mechanical properties and smoothness. The optimal dosage range identified in this study (0.8–1.0%) appears to strike a balance, providing acceptable mechanical recovery without exceeding the reference properties.

These findings emphasize the complexity of wet-end papermaking, where numerous factors, including fiber type, stock preparation, and additive interactions, must be carefully managed. In conclusion, while the use of retention agents offers significant benefits for papermaking processes, their impact on recycled paper properties, such as tensile strength and smoothness, cannot be overlooked. Striking the right balance between process efficiency and product quality remains a key challenge, requiring ongoing research and collaboration between industry and academia.