HIGH-SHEAR RHEOLOGY OF PAPER COATING COLORS - MORE THAN JUST VISCOSITY

Capillary viscometry is used to characterize viscosity, entrance press ure loss and apparent wall slip of paper coating colors at high shear rates. Special emphasis is laid on the dependence of these phenomena o n solids content in order to account for changes in the rheology due t o the dewatering of the color during the coating process. Coating colo rs with substantially different runnability have been investigated. Di fferences in apparent wall slip and high shear viscoelasticity (manife sting itself in extremely high entrance pressure losses) are observed at increased concentration, even if these phenomena do not show up at the initial solids content. Poor runnability is observed when viscosit y, entrance pressure loss and wall slip increase strongly with increas ing solids content. But all rheological features change simultaneously with the coating color recipe and it is not possible to separate out the contribution of the particular rheological features on the runnabi lity of the coating colors or to correlate the runnability to a single rheological parameter. Future work will have to focus on a numerical analysis of the blade coating process taking into account all the rheo logical features described here. First simulations including slip at t he color/blade interface indicate that wall slip may cause severe runn ability problems, at least when the apparent slip velocity exceeds the web velocity.