Introduction: The development of sustainable packaging as an alternative to petroleum-based materials has become one of the most important research priorities. Paper is a suitable option for sustainable packaging due to its recyclability and biodegradability, but the hydrophilic nature of cellulose fibers and their porous structure limit the desirable barrier properties. Therefore, the use of bio-based coatings has been considered as an alternative to petroleum-based coatings. In this study, the surface of handsheet was modified using a layer-by-layer (LbL) method using chitosan (CS) and carboxymethyl cellulose-precipitated calcium carbonate (CMC–PCC) complex, and the effect of the number of coating layers on the structural, mechanical, and barrier properties of the paper was investigated.
Method: First, bleached long fiber pulp with a freeness of 750 ml CSF was refined to 350 ml CSF using a PFI mill. Then, the pulp suspension (2% consistency) was treated with CMC and CaCl₂ solutions at boiling-water temperature for 2 h. The treated fibers (30% wt) were used to prepare handsheets with a grammage of 60 g/m². To modify the surface, the papers were first immersed in a chitosan solution (2.5 mg/mL) for 10 min, then rinsed with purified water. Next, the samples were immersed in a CMC-PCC complex mixture at 1.5 mg/mL for 10 min and rinsed again. This cycle was repeated to produce one-, two-, and three-bilayers. Morphological characteristics, particle size and zeta potential, thickness and grammage, tensile index, air permeability, water contact angle, water absorption time, brightness, and opacity were measured.
Results: Scanning electron microscope (SEM) images showed that coating with CS–CMC/PCC reduced the observable void spaces on the paper surface. The presence of CMC as a stabilizer prevented the sedimentation of PCC particles and increased their adsorption onto the fiber surface.
Zeta potential measurements confirmed the negative charge of the CMC-modified fiber suspension (−102.3 mV) and the CMC–PCC complex (−5.7 mV), as well as the positive charge of chitosan (+13.2 mV), which provides the necessary condition for the LbL process. The results showed that paper thickness and grammage increased significantly with increasing bilayer number. Despite the presence of the mineral filler, the tensile index increased by 4.25%, 10.89%, and 19.52% in the one-, two-, and three-bilayer samples, respectively, compared with the control sample, which was attributed to the formation of stable layers through electrostatic interactions and the reinforcement of the hydrogen-bond network among chitosan, CMC, and cellulose. Air permeability resistance increased by 272.66%, 652.62%, and 1319.05% in the one-, two-, and three-bilayer samples, respectively, confirming a considerable reduction in the void space between fibers. The water contact angle in the three-bilayer sample increased from 18.31° in the control sample to 81.50°; moreover, the water absorption time of this sample was also higher than that of the control sample. Treatment with one-bilayer of CS–CMC/PCC increased the brightness of paper due to the presence of CMC and PCC, but with an increase in the number of layers, the dominance of chitosan caused a decrease in brightness; nevertheless, layer-by-layer modification resulted in a considerable improvement in paper opacity.
Conclusion: The LbL method using chitosan and the CMC–PCC complex is proposed as an effective, biocompatible, and controllable approach for the simultaneous modification of the surface, mechanical, barrier, and optical properties of handsheets. Increasing the number of bilayers from one to three-bilayers resulted in the improvement of tensile index, air resistance, water contact angle, water absorption time, and opacity.