N. Willenbacher et al., HIGH-SHEAR RHEOLOGY OF PAPER COATING COLORS - MORE THAN JUST VISCOSITY, Chemical engineering & technology, 20(8), 1997, pp. 557-563
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.