Application of pigmented coating colours containing MFC/NFC: Coating properties and link to rheology

Micro (MFC) and nanofibrillar (NFC) cellulose materials exhibit strong attraction for water, with the finest fraction acting as superabsorber, swelling to form a gel. This water retaining behaviour has recently been studied in paper and board coating formulations as a replacement for the traditional water retention aid/thickener function of polymers, such as carboxymethyl cellulose (CMC) or polyacrylic salts. It is seen that the gel-like structures formed using MFC/NFC are distinctly different to those of the traditional particle flocculating properties of CMC, confirming previous findings that viscoelasticity is at a similar level to that of CMC containing colours, but that the recovery of elasticity after shear is slower. This study undertakes to formulate coating colours adopting MFC/NFC as water retention aid, and to evaluate coated board properties in relation to their rheological behaviour. The coatings are applied as double layers of the same formulation, avoiding the likely sagging effect resulting from the slower structure recovery but, in turn, maximising the benefits seen from extended levelling of the topcoat in the case where holdout is guaranteed by the presence of the precoat acting to block substrate voids. The net benefit of extended levelling is particularly observed with the coating formulation containing MFC as water retention aid.