A polymer coating of 6:0 mm thickness is cast onto a nonporous flat surface. The coating contains a residual amount of casting solvent, which is uniform at 1.0 wt% within the coating. The mass transfer of solvent through the polymer coating is controlled by molecular diffusion. The air flowing over the coating surface eliminates convective mass-transfer resistances and reduces the solvent vapor concentration in the air to nearly zero. The effective diffusion coefficient of the solvent molecules in the polymer is 2 x 10^-6 cm^2/s.
a. How many hours will it take for the solvent concentration at 1.2 mm from the surface to be reduced to 0.035 wt%?
