Cellulose Nanofibers Use in Coated Paper

Cellulose Nanofibers (CNF) are materials that can be obtained by the mechanical breakdown of natural fibers. CNF have the potential to be produced at low cost in a paper mill and may provide novel properties to paper, paper coatings, paints, or other products. However, suspensions have a complex rheology even at low solid contents. To be able to coat, pump, or mix CNF at moderate solids, it is critical to understand the rheology of these suspensions and how they flow in process equipment; current papers only report the rheology up to 6% solids. Few publications are available that describe the coating of CNF onto paper or the use of CNF as an additive into a paper coating. The rheology of CNF suspensions and coatings that contain CNF were characterized with parallel-disk geometry in a controlled stress rheometer. The steady shear viscosity, the complex viscosity, the storage modulus, and the yield stress were determined for the range of solids or concentrations (2.5-10.5%). CNF were coated onto paper with a laboratory rod coater, a size press and a high speed cylindrical laboratory coater (CLC). For each case, the coat weights were measures and the properties of the papers were characterized. CNF water base suspension was found to be a shear thinning with a power law index of around 0.1. Oscillatory tests showed a linear viscoelastic region at low strains and significant storage and loss moduli even at low solids. The Cox Merz rule does not hold for CNF suspensions or coating formulations that contain CNF with complex viscosities that are about 100 times larger than the steady shear viscosities. Paper coating formulations that contain CNF were found to have viscosities and storage and loss moduli that are over ten times larger than coatings that contain starch at similar solids. CNF suspensions were coated on papers with low amount transferred on paper either at high solids or high nip loadings. The amount transferred appears to be controlled by an interaction of filtration and fluid flow mechanisms. Coatings with CNF of 5 pph (part per hundred) were not able to be applied with CLC coater due to solid like behavior of the coating: the issue seems to be the ability of the coating to flow in the pond by gravity to the blade-paper nip and not the flow in the blade region itself. Some improvement of the paper properties were found by coating CNF onto paper, but some improvements were much less than expected. This may be due to CNF soaking into the paper. Air permeability decreased and stiffness increased. Paper coatings with CNF had higher stiffness and coating strength than coatings that contained starch. CNF acts similar to starch in terms of being a co-binder. A decrease in pick resistance is seen at 5 pph CNF or starch content; this decrease likely comes from the coating layer becoming more brittle