Effects of rheological property of coating liquid and withdrawal velocity on dip coating process in manufacturing of capsules

Abstract

In the present paper, to elucidate effects of the formation and breakage of filament on the dip coating process in the manufacturing of capsules, we measured the filament length between the tip of the coated rod and the liquid surface when the filament broke, and examined the dependence of the filament length on the Theological properties of coating liquids and withdrawal velocity to optimize the dip coating process. The coating liquids were aqueous solutions of syrup as Newtonian liquids and mixture liquids of aqueous solutions of syrup and water-soluble polymer (polyacrylamide) as polymeric liquids.Consequently, it is found that a large withdrawal velocity brings a shorter time at the filament breakage in Newtonian liquids regardless of their shear viscosities. The experimental equation which represents the relation between the breakage length of the filament and the shear viscosity of the coating liquid are presented. In polymeric liquids, a large elongational viscosity causes a large filament length.The large withdrawal velocity was ineffective upon the reduction of the breakage time of the filament. The use of polymeric liquids with large spinnability reduces the efficiency of manufacturing in dip coating process. The normal stress difference of the coating liquid reduces the coating thickness, so that the coating liquid containing polymer is required to have a small spinnability and a moderate normal stress difference.