Using slit rheometry in characterising coating colour flow in the blade coating process

摘要

The aim of the present work was to evaluate the suitability of SLIT rheometry for characterising the coating colour flow in paper coating. The detailed aims were to determine the mechanisms governing the flow through the SLIT; to find out what kind of information on the coating colour flow that can be obtained with the SLIT; and to evaluate the ability of the SLIT to predict the relationship between coat weight and blade pressure.For this purpose, flows of Newtonian model fluids and coating colours were studied both with conventional capillary and SLIT geometries attached to a high-shear rheometer. Correlations with laboratory coating data were determined for both geometries.It was found that the flow through the SLIT does not attain a fully developed velocity profile and is unstable along the whole length of the SLIT. The instability caused by entrance effects is present at the exit from the SLIT as residual entrance pressure.The SLIT geometry was found to be more sensitive to differences in coating colour formulations and also able to differentiate between coating colours displaying similar flow behaviour in conventional capillary measurements.In a laboratory trial, the differences in coating formulations reflected by the SLIT measurements were found to be in good correlation with the blade position required to achieve the target weights. Coating colours demonstrating greater flow resistance in SLIT measurements required a smaller blade distance to achieve the required coat weight.