Micro- and Nanofibrillated Cellulose as a Rheology Modifier Additive in CMC-Containing Pigment-Coating Formulations

Suspension rheology of aqueous coatings influences the coating application performance at high speeds and during high rates of change of the shear rate, as well as the quality of the coated end product determined by the relationship between dewatering, immobilization, and coating coverage. In the case of paper coatings, the end-use printing can be significantly affected by the coating uniformity, pore structure, and surface chemistry. Nanocellulose-containing materials, such as microfibrillated (MFC) and nanofibrillated (NFC) cellulose, are potential additives that could at least partly substitute other natural and synthetic cobinders, including viscoelasticity-inducing starch, carboxymethyl cellulose (CMC), and polyacrylic thickeners, in paper-coating color formulations. Work is reported here in which a systematic comparison of dewatering and theological characteristics of coating colors, based on three different pigments and mixtures thereof, is illustrated using CMC as the cobinder and incorporated with MFC/NFC as the partial cobinder replacement. All colors are shown to exhibit viscoelasticity, but the MFC/NFC additives are seen to operate as water-binding gel-forming components rather than with the flocculating thickening action of CMC. Immobilization of the color in the presence of nanofibrillar material is identified as the point of gel-water entrapment rather than the traditional stick-slip particle-particle interlocking mechanism predominating in traditional flocculant thickener formulations.