Document Type : Pulp and Paper
Authors
1 wood
2 Master's degree in Wood and Paper Industries, Kaveh Paper Mill, Saveh, Iran
Abstract
Background and aim: The increasing reliance of the papermaking industry on recycled fiber furnishes, together with the progressive degradation of fiber quality through multiple recycling cycles, represents a major challenge in maintaining adequate strength performance in paper products. Reductions in fiber length, flexibility, and bonding capacity weaken inter-fiber bonding and consequently diminish the structural integrity of recycled-fiber papers. These issues are particularly critical in packaging grades that demand high mechanical strength. Chemical strengthening agents and fiber-reinforcement strategies offer potential solutions; however, the optimal dosages of cationic starch and alum, as well as the appropriate proportion of imported long fibers required to enhance sheet performance, remain insufficiently explored. This study therefore aimed to investigate the individual and interactive effects of these three additives on the physical and mechanical properties of handsheets produced from recycled pulp.
Materials and Methods: Recycled pulp supplied by Kaveh Paper Mill was refined to approximately 300 mL CSF and used as the base furnish. Cationic starch, following preparation and gelatinization, was added to the furnish at four dosage levels (0, 1, 1.5, and 2% based on oven-dry fiber). Alum (aluminum sulfate, 17% purity) with acidic pH was applied at three levels (0, 1.5, and 2%). Imported long fibers were repulped and blended with the recycled furnish at 0, 10, and 15%. Laboratory handsheets with a basis weight of 130 g/m² were produced according to TAPPI standards. Mechanical properties—including tensile strength, tear index, burst strength, Ring Crush Test (RCT), and Corrugated Medium Test (CMT)—were evaluated, along with physical properties such as water absorption and sheet thickness. Statistical analyses were conducted using a factorial experimental design with ANOVA and Duncan’s multiple range test.
Results: Cationic starch significantly improved all measured strength properties, with the 1.5% dosage yielding the greatest enhancement. This improvement is attributed to increased cationic charge density, enhanced fiber–fiber bonding, and densification of the fiber network through stronger hydrogen bonding. In contrast, alum at dosages above the optimal level reduced mechanical performance; the 2% treatment, in particular, produced marked decreases in tensile, tear, and burst strengths. These reductions may be linked to excessive alum deposition, charge reversal effects, and impaired inter-fiber bonding resulting in a more brittle fiber matrix.
The addition of imported long fibers led to substantial improvements in mechanical strength by contributing longer, more flexible, and more collapsible fibers to the network. The 15% addition provided the most pronounced reinforcement, especially when combined with 1.5% cationic starch, which yielded the highest overall strength values. In terms of physical behavior, cationic starch reduced water absorption and increased sheet density due to enhanced fiber consolidation, whereas alum increased water absorption owing to its hydrophilic nature. The incorporation of long fibers increased sheet thickness as a result of forming a bulkier fiber structure.
Conclusion: Overall, the findings demonstrate that an optimized combination of chemical additives and fiber reinforcement can significantly enhance the quality of papers produced from recycled pulp. The synergistic application of 1.5% cationic starch and 15% imported long fibers was identified as the most effective treatment for improving both mechanical strength and physical performance. These results offer practical guidance for improving the efficiency of recycled-fiber-based papermaking systems and reducing dependence on virgin fibers in industrial production.
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