Drying process paths of ternary polymer solution coating

Drying process paths of a ternary solution consisting of a polymer dissolved in a blend of a solvent and a nonsolvent are analyzed. A 1-D drying model applied to a ternary solution provides composition trajectories followed at different depths in the coating as a function of time, collectively called process paths. The effects of gas-phase mass-transfer coefficient, relative diffusivity and relative volatility, and partial saturation of the gas phase with solvents on these process paths are presented. When the solution coating enters the two-phase region, phase separation may take place, as illustrated by coatings prepared from ternary solutions of cellulose acetate in ketone and methanol. When acetone is the solvent, the coating phase separates. Replacing acetone with methyl ethyl ketone gives homogeneous coatings. Solution phase separation or "blush" is explained in terms of the drying-process paths and their relationship to the two-phase immiscible region.