Date of Award
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Doctor of Philosophy (PhD)
Second Committee Member
Third Committee Member
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During the blade coating of paper, operational issues such as stalagmite formation, scratches or spits can develop at high solids content and high web speeds. For coatings that contain high aspect ratio pigments, these difficulties appear at lower solids concentrations and slower machine speeds than with more spherical shaped pigments. These operational issues are important because plate shaped particles have good properties in publication and barrier grades. A number of possible mechanisms have been described in the literature, but a clear cause has not been established nor have methods to estimate the operational limits been proposed.
The influence of particle characteristics on rheology, dewatering and operational issues was determined with a series of experiments. The effect of particle shape and size distribution on the dewatering and filtercake permeability were analyzed. The porosity and pore size distribution of dried filtercakes were also characterized. Runnability studies were conducted to determine the operational windows based on pigment shape and solids content. A bench top blade coater was developed to measure operational issues; small changes in the roll speed resulted in fluctuations in the speed at which blade deposits begin to appear. A high speed cylindrical laboratory coater (CLC) was used to determine the operational and quality issues for various solids content for two pigments with different shape factors on two paper surfaces with varying absorption rates; a mobile video camera, mounted to the blade structure, was used to record the coating event.
The rate of dewatering of a coating formulation is influenced but the shape of the particles, where high aspect ratio pigments tend to dewater slower than those with a lower shape factor. High aspect ratio pigments have low filtercake permeabilities compared to the low aspect ratio pigments. The results indicate that the rate of dewatering is controlled by the size of the connections or throats between the pores and not the average pore size. Latex addition, in these systems, did not influence permeability to a large extent.
A method to determine the operational window was developed using the bench top blade coater which provides a measure of the maximum speed attainable at a given solids content before runnability defects appear: operational limits obtained with the bench coater correlate with those obtained with the CLC. With the absence of a base sheet, latex or other additives and binders, typical blade deposits were still generated with the bench coater. Small changes in solids content lead to large changes in coating speed. The influence of a permeable substrate on operational limits was minor. The operating window did not correlate with steady shear viscosity but did seem to relate to complex viscosity. A linear relationship between the operational limits and the difference between the coating solids and the immobilization solids. This relationship may help predict operational limits of other coating formulations. Operating limits seem to be determined by a dynamic flow leading to particle jamming behavior.
A mathematical model was developed to estimate the formation of a filtercake during blade coating. Experimental values for all parameters were based on the conducted runnability studies. The permeabilities of the base sheet and the coating filtercake play a crucial role in the formation of the filtercake. Using the conditions that are similar to the CLC experiments, it was found that filtercake growth would not have impeded blade coating.
Weeks, Lisa, "Operational Limits of Blade Coating with High Aspect Ratio Pigments" (2017). Electronic Theses and Dissertations. 2792.