Size-selective absorption and adsorption in anionic pigmented porous coating structures: case study cationic starch polymer versus nanofibrillated cellulose

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. The benefits of capturing the important NFC in a layer structure, to enhance surface and stiffness properties of paper, board and laminates whereby NFC is entrapped at the surface of a fibrous web by forming an in situ composite, were previously shown for the exemplified case of modified porous calcium carbonate, as might be used in an inkjet coating application (Ridgway and Gane in Cellulose 19(2):547–560, 2011). The NFC is seen to integrate itself within the larger interparticle 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. The concept need not be confined to porous pigments, as any pigment coating structure that absorbs and holds the NFC, thus creating an in situ composite, could be used. The aim of this study is to look at a range of different pigments and investigate how these could be used as coating structures by measuring the effect on the pore structure before and after absorbing NFC. This is achieved by using model porous tablet blocks made from the respective anionic coating formulations. The penetration of cationic starch solution, as might be applied for surface sizing on paper, is studied for comparison. The use of cationic starch is considered in the industry to provide reasonably effective surface concentrations due to the electrostatically driven adsorption to the anionic pore surfaces. The effect of water alone on the coating structure has also been measured to allow for structural relaxation, considered to be mainly related to the swelling properties of the anionic polyacrylic coating pigment dispersant. The results illustrate the size-exclusion properties of the pore structure in relation to the material being absorbed and partial resistance to bulk penetration by pore wall adsorption in the case of oppositely charged species. The distribution of the absorbate throughout the pore network can be derived using mercury intrusion porosimetry and electron microscopy, and is deemed critical in respect to controlling the end performance properties, be they, for example, barrier, strength-enhancing applications, or both.