Gravimetric analysis of membrane casting : I. Cellulose acetate — acetone binary casting solutions

Solvent evaporation from cellulose acetate solutions cast to a thickness of 100–500 μ m is followed gravimetrically at 12–28°C. The desolvation curves include a region in which ω (solvent/polymer weight ratio) decreases almost exponentially with time. The exponential decrease lasts, at 28°C, until 75–85% solvent has evaporated, whereas below 19°C, upward bending in the log w vs. t relation is observed after evaporation of about half the solvent. The time constant of desolvation, τ, increases with the initial acetone weight per unit area, d, as τ α d^γ, where γ = 1.4–1.6. About a threefold increase in τ is observed on decreasing temperature from 28 to 12°C. Numerical solutions of the diffusion equation are obtained by taking time-dependence of surface solvent concentration and movement of the solution/air interface into account. The experimental facts are shown to be explicable by comparison with calculations. The effectiveness of the gravimetric approach to an understanding, characterization, and control of membrane casting processes is discussed.