Highly Porous Paper Loading with Microfibrillated Cellulose by Spray Coating on Wet Substrates

A laboratory bench for the spray coating of aqueous slurries on wet substrates and the subsequent water removal by vacuum filtration was used to load porous papers with microfibrillated cellulose (MFC). Spray coating provided an accurate control of the coating basis weight and, when increasing the MFC load, homogeneous films progressively formed on the substrate with negligible loss of MFC. Even in the presence of porous papers, the MFC coating remained confined on the surface of the substrate, at first forming irregular pots and then, as the MFC basis weight exceeded 6 g/m(2), a continuous film. The formation of a MFC film induced a drop in the air permeability and a sharp increase in the tensile properties. Also, after film formation, the further increase of the MFC coating basis weight led to a linear increase of the tensile properties that, as for laminates, were predicted by the rule of mixtures.