The Role of Cellulose Nanofibrils in Structure Formation and Drying Stress Development of Pigment Coating Layer

Abstract

Coating structure is one of the important factors determining pigment coated paper quality including printability. The structure is influenced by the type or composition of pigment, binder, and additives. Even though the pigment and binder determine the structure of the coating layer most significantly, a small amount of additives can also influence the structure. In this study, cellulose nanofibrils (CNF) were used as an additive of the coating color. CNF was selected as an additive because it is a potential material to thicken the coating color. The effect of CNF on the coating color and coating layer was suggested compared to carboxymethyl cellulose (CMC) that has been used as a traditional thickener. Initially, the rheological properties of the coating color were investigated to understand the effect of CNF on the microstructure of the coating color. Drying kinetics was evaluated using the multispeckle-diffusing wave spectroscopy (MS-DWS) technique to suggest structure formation in the coating layer. Stress development was evaluated to investigate the effect of CNF on shrinkage of the coating layer. In addition, surface characteristics of the coating containing CNF were evaluated. Finally, CNF was applied to coated paper to investigate the change of in the optical, structural, and absorption properties of the coated paper.

CNF coating showed lower elastic behavior than CMC coating because there was no interaction between the particles in the coating color containing CNF. The microstructure of the coating color influenced the drying process. The particles in the CNF coating color moved freely because CNF did not form a structural network and increase the viscosity of the aqueous phase. The movement of the coating components, however, was greatly restricted by CMC because it created a network structure and increased the viscosity of the aqueous phase. CNF made the coating layer porous in a different manner than CMC. The voluminous characteristics of CNF made the coating layer porous.

CNF coating showed different stress development behavior compared to CMC. The drying stress of the coating layer increased with the addition of CMC because of the loosely packed structure of CMC coating, which increased the total shrinkage of the coating layer after the solidification point and the shrinkage of the precipitated CMC during drying. The stress of the CNF coating, however, was lower than that of the CMC coating because of the less structured CNF coating and the low shrinkage characteristics of cellulose. The CNF coating showed much lower gloss than the CMC coating due to its water absorbing characteristics. The swollen CNF caused a rough surface because the shrinkage of CNF proceeded until the end of the drying process.

The CNF coating gave lower coated paper gloss than that of the CMC coating due to non-uniform shrinkage, which came from the gel-like structure of CNF. To improve the gloss of the coated paper, the coated paper was made with a low coat weight and under high shear rate. The gloss and roughness of the coated paper were similar for the CNF and CMC coatings. In addition, the surface defects in the coated paper diminished. The CNF coating greatly improved ink absorption compared to CMC coating in the ink absorption test and the modified Vandercook press test by forming a porous coating layer due to its low shrinkage characteristics. The small amount of CNF addition can promote the absorption rate and uniformity of coated paper without changing the material properties.