Introduction: In the papermaking industry, mineral fillers are extensively used to reduce production costs and enhance the optical properties of paper. Precipitated calcium carbonate (PCC) is one of the most widely applied fillers due to its low cost, high availability, and excellent contribution to brightness and opacity. However, the major drawback associated with PCC application is its low retention within the fibrous network and the deterioration of paper strength properties. These limitations mainly result from weak interfacial interactions between PCC particles and anionic cellulose fibers, leading to filler loss during drainage and disruption of fiber–fiber bonding. To overcome these challenges, surface modification of PCC using biodegradable cationic polymers has been proposed as an effective strategy to improve filler–fiber compatibility and simultaneously enhance filler retention and paper performance. Therefore, the objective of this study was to investigate the effects of surface modification of PCC using cationic starch, chitosan, and their combination, as well as to evaluate the role of cationic polyacrylamide (CPAM) as a retention aid on the optical, structural, and mechanical properties of printing and writing paper.
Methods: In this study, PCC was surface-modified separately with cationic starch, chitosan, and a starch–chitosan blend. The modification process involved preparing PCC slurries, adding the cationic polymers under controlled pH and temperature conditions, followed by filtration, drying, and grinding of the modified fillers. Handsheets with a basis weight of 60 g/m² were produced using long-fiber pulp. In selected formulations, CPAM was added as a retention aid. The handsheets were evaluated according to TAPPI standard methods. Ash content was measured as an indicator of filler retention, while brightness, opacity, and apparent density were determined to assess optical and structural properties. Mechanical performance was evaluated by measuring tensile strength index, burst strength index, and tear strength index.
Results: The results demonstrated that surface modification of PCC with cationic starch, compared to chitosan, yielded superior performance in simultaneously enhancing filler retention and mechanical properties. Although the addition of cationic polyacrylamide significantly increased filler retention in both systems (starch and chitosan), cationic starch showed a marked advantage due to its greater stability and the formation of more effective bonds with the fibers, particularly regarding strength properties (tensile and burst strengths). This improvement was attributed to strong electrostatic attractions and hydrogen bonding between starch-modified PCC particles and cellulose fibers, which enhanced filler entrapment within the paper structure. In addition, starch-modified PCC significantly improved optical properties, including brightness and opacity. Notably, this treatment also led to substantial improvements in mechanical properties, effectively compensating for the strength losses typically associated with filler addition.In contrast, PCC modified solely with chitosan exhibited limited performance. This behavior was mainly attributed to process-related limitations of chitosan, such as partial loss of cationic charge under near-neutral pH conditions and deactivation during acidic washing steps, which reduced its effective adsorption onto PCC surfaces. The combined use of starch and chitosan did not provide a significant advantage over cationic starch alone, indicating that the performance of this system was largely governed by starch. Although CPAM generally enhanced filler retention, its simultaneous use with multiple cationic polymers occasionally caused excessive flocculation, leading to irregular filler aggregation and reduced mechanical strength.
Conclusion: Overall, the findings of this study indicate that surface modification of PCC with cationic starch is a highly efficient, biodegradable, and reliable strategy for improving filler retention as well as the optical and mechanical properties of printing and writing paper. In contrast, the effective application of chitosan requires careful control of processing conditions to maintain its cationic functionality. These results provide valuable insights for the development of sustainable and optimized filler modification strategies in modern papermaking.