ZnOnp/CaCO3 Core–Shell Nanoparticle Coatings on Kraft Paper: A Comparative Study of Antimicrobial Efficacy, Tensile Strength, and Hydrophobicity

This study introduces a novel paper coating approach using modified zinc oxide (ZnO), providing a comparison with conventional materials used in the paper industry. The research focused on determining the concentration for effective microbial growth inhibition and evaluates the impact of different ZnO types on coated-paper properties, including antimicrobial activity, surface morphology, tensile strength, and water absorption. Specifically, ZnO microparticles (ZnOws), ZnO nanoparticles (ZnOnp), and modified ZnOnp (ZnOnp-CaCO3, with a core–shell structure composed of calcium carbonate [CaCO3] and nano-zinc oxide) were incorporated into coating formulations at varying concentrations (0 × MIC, 1 × MIC, 2 × MIC, and 3 × MIC, based on minimum inhibitory concentrations [MICs]). The results demonstrated that among all tested microorganisms, ZnOnp-CaCO3 showed the lowest MIC values. ZnOnp-CaCO3-coated paper exhibited superior antimicrobial activity against both Gram-positive and Gram-negative bacteria, as well as fungi, outperforming ZnOws and ZnOnp. At 1 × MIC, %inhibition for E. coli, S. aureus, and A. niger were 98.3%, 99.1%, and 90.8%, respectively. Additionally, ZnOnp-CaCO3 coatings caused minimal color change in the paper compared to the other ZnO variants. The coating did not negatively impact the mechanical properties of the paper across all ZnO types and concentrations. Water absorption tests showed increased hydrophobicity with higher ZnO content, with ZnOnp and ZnOnp-CaCO3 exhibiting greater reductions in water absorption than ZnOws. Overall, ZnOnp-CaCO3 showed strong potential as an antimicrobial agent for paper surfaces, making it ideal for packaging and hygiene products. By partially replacing ZnOnp with inexpensive CaCO3 core particles, ZnOnp-CaCO3 delivers enhanced performance, reduced costs, and greater sustainability for large-scale applications.