Constructing NFC-pigment composite surface treatment for enhanced paper stiffness and surface properties

The use of nano- or microfibrillar cellulose (NFC or MFC) in papermaking is generally hampered by high cost and potentially wasteful use in typical wet end applications. The solubility and fines nature of the material makes it inefficient to retain, and when retained it is generally inefficiently applied within the spatial distribution of the paper fibre matrix. To illustrate the benefits of capturing the important NFC in a layer structure to enhance surface and stiffness properties of paper and board, we present a study whereby NFC is entrapped at the surface of a fibrous web by forming an in situ composite using a porous coating layer, consisting in the exemplified case of modified calcium carbonate. It is shown that NFC can integrate itself within the porous structure providing excellent holdout and thin layer continuity essential in developing an efficient concentration of the NFC at the surface of the substrate. The effect is likened to the well-known I-beam construction. An additional feature is the potential for recycling the remaining fibrous content in the NFC or, more particularly, MFC product after the nanocrystalline cellulose (NCC) gel fraction has been absorbed, allowing for further efficient processing if needed and hence providing a potential cost reduction in the overall NFC/MFC production. The increased smoothness and uniformity obtained is illustrated by confocal laser profilometry and electron microscopy. The effect on permeability is also illustrated.