# Impact of Retention Agents on Functional Properties of Recycled Paper in Sustainable Manufacturing  
[PERSON]  
1 Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences--SGGW, 159 Nowouysynowska Str., 02-787 Warsaw, Poland; [EMAIL_ADDRESS]; Tel. : +48-22-59-385-45 Natural Fibers Advanced Technologies, 42A Blekina Str., 93-322 Lodz, Poland12 Natural Fibers Advanced Technologies, 42A Blekina Str. 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 (Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)) [10], polyaluminum chloride (PAC) [11], and ferric chloride (FeCl\({}_{3}\)) [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.