The effect of yielding of dense silica slurry on the uniformity of coated layer

The effect of shearing conditions in the coating process on the uniformity of the coated layer of dense silica slurry is investigated. The pH 6.8 test slurry mainly used in this study forms a nonuniform coated layer only on the condition of large shear strain and low shear rate. The same shearing dependency was observed using not only a laboratory-scale doctor blade but also a pilot-scale comma coater. The test slurry exhibits two-step yielding, and the internal structure of the slurry is gradually destroyed after the second yielding. When the shear strain applied is within the second yielding, the test slurry is in equilibrium and a uniform coated layer is formed. As strain is increased further, the slurry changes from the viscoelastic fluid to Newtonian. When the shear strain is beyond the second yielding but the structural destruction is not completed, a nonuniform coated layer is formed. A higher coating speed or sufficiently large shear deformation, which effectively destroys the internal structure, is favorable to obtain a uniform coated layer.